A more recent version of these Defences notes – written by Oxford students – is available here.
The following is a more accessble plain text extract of the PDF sample above, taken from our Criminal Law Notes. Due to the challenges of extracting text from PDFs, it will have odd formatting:
DEFENCES Even if the prosecution has proved the AR and MR of an offence, the D may still be able to argue that he has a defence. SELF-DEFENCE The requirements of the defence are:
1. D was (or believed he was) facing an unjust threat from the V.
2. The D used a level of force against the threat (or the threat as he belived it to be) that was reasonable in the circumstances. s.3 Criminal Law Act 1967 (1) "A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large." (2) "Subsection (1) above shall replace the rules of the common law on the question when force used for a purpose mentioned in the subsection is justified by that purpose." The defence was further clarified in the following statute: s.76 Criminal Justice and Immigration Act 2008 (1) This section applies where in proceedings for an offence---
(a) an issue arises as to whether a person charged with the offence ("D") is entitled to rely on a defence within subsection (2), and (b) the question arises whether the degree of force used by D against a person ("V") was reasonable in the circumstances. (2) The defences are---
(a) the common law defence of self-defence; (aa) the common law defence of defence of property; and (b) the defences provided by section 3(1) of the Criminal Law Act 1967 (c. 58) or section 3(1) of the Criminal Law Act (Northern Ireland) 1967 (c. 18 (N.I.)) (use of force in prevention of crime or making arrest). (3) The question whether the degree of force used by D was reasonable in the circumstances is to be decided by reference to the circumstances as D believed them to be, and subsections (4) to (8) also apply in connection with deciding that question. (4) If D claims to have held a particular belief as regards the existence of any circumstances
(a) the reasonableness or otherwise of that belief is relevant to the question whether D genuinely held it; but (b) if it is determined that D did genuinely hold it, D is entitled to rely on it for the purposes of subsection (3), whether or not---
(i) it was mistaken, or (ii) (if it was mistaken) the mistake was a reasonable one to have made.
(5) But subsection (4)(b) does not enable D to rely on any mistaken belief attributable to intoxication that was voluntarily induced. (5A) In a householder case, the degree of force used by D is not to be regarded as having been reasonable in the circumstances as D believed them to be if it was grossly disproportionate in those circumstances. (6) In a case other than a householder case, the degree of force used by D is not to be regarded as having been reasonable in the circumstances as D believed them to be if it was disproportionate in those circumstances. (6A) In deciding the question mentioned in subsection (3), a possibility that D could have retreated is to be considered (so far as relevant) as a factor to be taken into account, rather than as giving rise to a duty to retreat. (7) In deciding the question mentioned in subsection (3) the following considerations are to be taken into account (so far as relevant in the circumstances of the case)---
(a) that a person acting for a legitimate purpose may not be able to weigh to a nicety the exact measure of any necessary action; and (b) that evidence of a person's having only done what the person honestly and instinctively thought was necessary for a legitimate purpose constitutes strong evidence that only reasonable action was taken by that person for that purpose. (8) (8) Subsections (6A) and (7) are not to be read as preventing other matters from being taken into account where they are relevant to deciding the question mentioned in subsection (3). (8A) For the purposes of this section "a householder case" is a case where---
(a) the defence concerned is the common law defence of self defence, (b) the force concerned is force used by D while in or partly in a building, or part of a building, that is a dwelling or is forces accommodation (or is both), (c) D is not a trespasser at the time the force is used, and (d) at that time D believed V to be in, or entering, the building or part as a trespasser. (8B) Where---
(a) a part of a building is a dwelling where D dwells, (b) another part of the building is a place of work for D or another person who dwells in the first part, and (c) that other part is internally accessible from the first part, that other part, and any internal means of access between the two parts, are each treated for the purposes of subsection (8A) as a part of a building that is a dwelling. (8C) Where---
(a) a part of a building is forces accommodation that is living or sleeping accommodation for D, (b) another part of the building is a place of work for D or another person for whom the first part is living or sleeping accommodation, and (c) that other part is internally accessible from the first part, that other part, and any internal means of access between the two parts, are each treated for the purposes of subsection (8A) as a part of a building that is forces accommodation. (8D) Subsections (4) and (5) apply for the purposes of subsection (8A)(d) as they apply for the purposes of subsection (3).
(8E) The fact that a person derives title from a trespasser, or has the permission of a trespasser, does not prevent the person from being a trespasser for the purposes of subsection (8A). (8F) In subsections (8A) to (8C) --- "building" includes a vehicle or vessel, and; "forces accommodation" means service living accommodation for the purposes of Part 3 of the Armed Forces Act 2006 by virtue of section 96(1)(a) or (b) of that Act. (9) This section, except so far as making different provision for householder cases, is intended to clarify the operation of the existing defences mentioned in subsection (2). (10) In this section---
(a) "legitimate purpose" means---
(i) the purpose of self-defence under the common law, (ia) the purpose of defence of property under the common law, or (ii) the prevention of crime or effecting or assisting in the lawful arrest of persons mentioned in the provisions referred to in subsection (2)(b); (b) references to self-defence include acting in defence of another person; and (c) references to the degree of force used are to the type and amount of force used. In the following case, the court explained that the statute does not change the law, rather sets out the existing position in statutory terms: Keane 
Facts: see below. Court of Appeal:
? S.76 of the Criminal Justice Immigration Act 2008: does not alter the law as it has been for many years; not does it exhaustively state the law of self-defence, but it does state the basic principles. To what crimes is self-defence a defence?
The offence against which you use the defence must involve force: Blake v DPP 
Facts: a vicar wrote with a felt-tip pen on a pillar near the Houses of Parliament. He argued that he was seeking to prevent the first Iraq war and therefore relied upon s.3 of the Criminal Law Act 1967. Held: the writing did not involve the use of force and therefore the defence could not be relied upon. What needs to be shown to establish the defence?
The key elements of the offence are as follows:
1. V must pose an unjustified threat
2. The use of force must be necessary
3. The degree of force must be unreasonable
4. The D must be acting in order to defend himself or another or property.
1. The victim must pose a threat The V must pose a risk to D or someone else. However, this requirement has been cast into doubt by the following case: Hitchens 
Facts: D was charged with an assault on Kathleen Brown. His defence was that he acted in self-defence, in that he assaulted her in order to prevent her allowing a man into her flat, who D believed would assault him. The judge ruled that a D could not rely on self-defence in a case where the assault was against an innocent person in order to prevent an attack against a third party. Court of Appeal:
? The defences as capable of extending the use of force against an innocent third party to prevent a crime being committed by someone else. This rule has greater scope for operation where it was certain that a crime would be committed immediately if action was not taken. Conversely, a low likelihood of a crime being committed, and a long time between awareness of risk and when the crime might be committed, would reduce the scope.
? However, in this case, V had been doing what she was entitled to do, there was little risk of crime being committed, and none was being committed when D slapped V. D had ample time to call the police given that he knew O was coming. Gross LJ:
? Although we suspect that the facts capable realistically of giving rise to such a defence will only rarely be encountered, examples can be adduced and two will suffice:
1. a police constable bundles a passerby out of the way to get at a man he believes about to shoot with a firearm or detonate an explosive device;
2. Y seeks to give Z car keys with Z about to drive. X, believing Z to be unfit to drive through drink, knocks the keys out of Y's hands and retains them. Herring: the CA have got this wrong. In the two hypothetical cases, the correct defence would be duress. The case threatens to undermine the line the law draws between duress and self-defence. There is no doubt that the defence applies whether the threat is being posed to the D or to another person / property. [Note: another argument is that this lets in the defence of necessity to murder in through the back door --- i.e. the only circumstances in which it would be reasonable for D to kill an innocent V would be where it was necessary.]
2. the use of force must be necessary It must be shown that it was necessary to use force; this means that it must have been reasonable for the D to use force, rather than to attempt to escape the situation in some way. NB: The D can make a 'pre-emptive' strike, it it was reasonable to do so: AG's Reference No. 2 of 1983 
Facts: D was a shopkeeper. His shop was in the centre of extreme riots --- PS600 worth of damage was done to his shop and PS400 worth of his goods were stolen. After this attack he remained in his shop without sleep and in fear of further attack. He made 10 petrol bombs to protect himself in case he was subject to further attacks. He was charged with offences under section 4 of the Explosive Substances Act 1883. He was acquitted, but the AG referred the question as to whether the defence of self defence could be used to cover preparing for attack. Court of Appeal: The defence could be allowed for offences based on possession in preparation of attacks provided the possession ceases when the danger of attack is no longer imminent. Lord Lane CJ:
? "The defendant in this case said that his intentions were to use the petrol bombs purely to protect his premises should any rioters come to his shop. It was accordingly open to the jury to find that the defendant had made them for the reasonable protection of himself and his property against this danger"
? "In our judgment a defendant is not left in the paradoxical position of being able to justify acts carried out in self-defence but not acts immediately preparatory to it. There is no warrant for the submission on behalf of the Attorney General that acts of selfdefence will only avail a defendant when they have been done spontaneously. There is no question of a person in danger of attack "writing his own immunity" for violent future acts of his. He is not confined for his remedy to calling in the police or boarding up his premises."
? "He may still arm himself for his own protection, if the exigency arises, although in so doing he may commit other offences. That he may be guilty of other offences will avoid the risk of anarchy contemplated by the Reference. It is also to be noted that although a person may "make" a petrol bomb with a lawful object, nevertheless, if he remains in possession of it after the threat has passed which made his object lawful, it may cease to be so. It will only be very rarely that circumstances will exist where the manufacture or possession of petrol bombs can be for a lawful object."
3. The use of force must be reasonable
It is now necessary to distinguish 'householder' cases from non householder cases. Normal cases the D can use the defence only if the level of force was reasonable in the face of the threat as perceived by the D. It should be stressed that the question is whether a reasonable person would say that the level of force was reasonable, not whether the D thought the level of force was reasonable. Objective test. Some guidance was given in the following case (NB: this was pre-2008, so before there was a split between homeowners and non homeowners R v Martin (Anthony) 
Facts: D lived alone on an isolated farm, which was subject to numerous break ins. The Vs broke into the farm on one such occasion; D, armed with a 12 bore shot gun, went downstairs and fired indiscriminately at where he thought the disturbance came from. He killed one of the Vs. D was convicted of murder. On appeal, his counsel sought to introduce new evidence that the D was suffering from a psychiatric condition which was relevant to show: (a) there was diminished responsibility; (b) because of the condition he was more likely to genuinely believe that he was about to be attacked than the ordinary person would be. Court of Appeal: Whilst the disorder could not be considered for the purposes of self defence, there was DR, so the murder conviction should be quashed. Lord Woolf CJ:
? What was the reasonable amount of force? "as to what is a reasonable amount of force, obviously opinions can differ. It cannot be left to a defendant to decide what force it is reasonable to use because this would mean that even if a defendant used disproportionate force but he believed he was acting reasonably he would not be guilty of any offence." Rather, it is for the jury to decide the amount of force "which it would be unreasonable to use in the circumstances in which they found that Mr Martin believed himself to be in. It is only if the jury are sure that the amount of force which was used was unreasonable that they are entitled to find a defendant guilty if he was acting in self-defence."
? On whether mental illness can be taken into account: He recognises that "too generous an approach as to what is reasonable could result in an exorbitant defence ...
We would accept that the jury are entitled to take into account in relation to self-defence the physical characteristics of the defendant. However, we would not agree that it is appropriate, except in exceptional circumstances which would make the evidence especially probative, in deciding whether excessive force has been used to take into account whether the defendant is suffering from some psychiatric condition." Most commentators (see Leverick) seem to think that the level of force applied can be greater than that threatened (e.g. killing in defence to rape); however, where the level of force is far
greater, there will be no defence. NB: the statute tells us that it's not an exact science --- it can't be weighed "to a nicety."
Householder cases s.76(5A): In a householder case, the degree of force used by D is not to be regarded as having been reasonable in the circumstances as D believed them to be if it was grossly disproportionate in those circumstances. A householder case will be one where the V is entering a building as a trespasser and the D is not a trespasser. In these cases, the force will only be unreasonable if it is "grossly disproportionate." Herring suggests that this will not effect much change (the section was amended in 2013) because juries used to take into account such 'emergency' home invader situations anyway.
4. The D must be acting in order to defend himself or another or property Note that this intersects with the Dadson principle --- the D cannot rely on the justifying circumstances of his / her actions of which he or she is not aware. So, if D was acting in selfdefence, but did not believe he was then he will have no defence. What about Ds who think they are being attacked, but are not?
The jury must ask whether, on the facts as perceived by the D, the level of force was reasonable. This was established in the following cases: Williams 
Facts: M saw a youth rob a woman in the street. M caught him again and knocked him to the ground. D, who had only seen the latter stages of the incident, was told by M that M was arresting Y for mugging a woman. M said he was a police officer, which was untrue, so could not produce a warrant card when D requested it. D then assaulted M by punching him in the face and was charged with assault occasioning ABH. D's defence was that he honestly believed Y was being unlawfully assaulted by M. Court of Appeal: if D was acting under a mistake as to the facts, he was to be judged according to his mistaken view of the facts --- it did not matter if the mistake was, objectively, reasonable. The reasonableness of D's belief was evidence as to whether the belief was truly held by him. Lord Chief Justice:
? What then is the situation if the defendant is labouring under a mistake of fact as to the circumstances? He must then be judged against the mistaken facts as he believes them
Buy the full version of these notes or essay plans and more in our Criminal Law Notes.