R v Rose [1884] the defendant who had shot dead his father whilst the latter was threatening murderous attack on the defendant's mother, was acquitted of murder on the grounds of self defence

c.s3(1) Criminal Law Act 1967 provides that : "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"

if self defence is successful it will result in the defendant being acquitted

• force in self defence will be lawful if used to repel, resist or ward off an attack or immediate threat

• the test for necessity is subjective: D is judged on the facts as he honestly believed them to be

• the amount of force must be reasonable according to D's subjective belief

the need for force is assessed to D's honest belief : this part is, therefore, a subjective test

the defence will not fail is D inflicts harm under a mistaken belief that he is under attack : the mistake must be genuine and honest but need not be reasonable

Palmer v The Queen (R) [1971] Lord Morris : "if there has been an attack so that defence is reasonably necessary it will be recognised that a person defending himself cannot weigh to a nicety the exact measure of his necessary defence action. If a jury thought that in a moment of unexpected anguish a person attacked had only done what he honestly and instinctively thought was necessary that would be most potent evidence that only reasonable defensive action had been taken"

R v Williams (Gladstone) [1984] a thief who robbed a woman in the street was apprehended and knocked down by a bystander M
T called for help, and D (who had not seen start of incident) intervened and hit M in order to protect T from further beating
at D's trial on charges of assault causing actual bodily harm, the recorder said D's mistaken belief that M was acting unlawfully would be a defence only if it was reasonable
Court of Appeal overruled this direction and said that any honest mistake would be sufficient
the reasonableness or unreasonableness of D's belief, said Lord Lane CJ, is material to the question whether the belief was held by D at all, but if the belief was in fact held its unreasonableness is irrelevant

• the amount of force used must be reasonable in all circumstances as perceived by D (semi objective)
• as a general rule the force used in self defence must be roughly commensurate with the threatened force
• reasonable force used is judged objectively but based on the subjective perception of the D as to the threat

R v McInnes [1971] Edmund Davies LJ : "but where self defence fails on the ground that force used went clearly beyond that which was reasonable in the light of the circumstances as they reasonably appeared to the accused, is it the law that the inevitable result must be that he can be convicted of manslaughter only, and not of murder? it seems that in Australia that question is answered in the affirmative ... but not, we think, in this country. On the contrary, if a plea of self defence fails for the reason stated, it afford the accused no protection at all"