D (wife) deserted P and they made an agreement that in return for some maintenance money, that P would pay each week, D would not claim for any more, pledge the husband’s credit etc. P argued that there was no consideration to pay the money since the wife was already legally incapable of claiming for more money while in desertion. CA allowed D’s claim
Lord Denning: a pre-existing duty (here, not to sue for more money) can give rise to consideration where it is not against public policy. In this case although the wife was unable to claim for more money in court anyway (i.e. her forbearance was a pre-existing duty), her refraining from doing so meant that the man did not have to go to court to defend himself and so she did confer a benefit on him .Also, since D could end the desertion, she would have been able to claim for more money upon returning to P, had it not been for this agreement. Thus there was consideration and D had to pay.
Other 2 judges concurred but not on the same basis: they only used Denning’s second argument.