Principle If a person voluntarily becomes or remains associated with others engaged in criminal activity in a situation where he knows or ought reasonably to know that he may be the subject of compulsion by them or their associates, he cannot rely on the defence of duress. Note that the case also contains important obiter statements in relation to many of the other elements to the defence.

The parents of six-week-old conjoined twins, Mary and Jodie, appealed against a ruling granting medical staff authority to proceed with surgical separation. Mary had severe brain abnormalities, no lung tissue, and no properly functioning heart. The blood supply keeping Mary alive emanated from who was in all other essential respects functioning and developing normally.

The doctors’ acts of separation would not amount to murder because the act was required to avoid inevitable and irreparable evil; no more would be done than was reasonably necessary for the purpose to be achieved, and the evil to be inflicted was not disproportionate to the evil avoided.

D saw a third party, V, arresting a youth, X. V said he was a police officer but failed to produce a warrant card. D wrongly thought that X was being unlawfully assaulted by V, and D assaulted V.

The Court of Appeal quashed D’s conviction. Where a defendant made a mistake about the need to use force (to defend himself or another), he was to be judged according to his mistaken view of the facts, whether or not that mistake was reasonable. This is a subjective test.