The general principles is that only such force may be used as is reasonable in the circumstances. This is a question for the jury. However, although the question of what is reasonable is to be judged by the jury, it is critical that they put themselves in the circumstances which the defendant supposed (whether reasonably or not) to exist. The jury should be reminded also not to disregard the state of mind of the defendant altogether. This recognises that the force will commonly be used in moment of crisis.
In the Privy Council case of palmer (1971), Lord Morris said that A person defending himself cannot weigh to a nicety the exact measure of his,,, defensive 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 the most potent evidence that only reasonable defensive action had been taken. The court of appeal has affirmed this passage. In whyte (1987) Lord Lane CJ said that "in most cases ,,, the jury should be reminded that D's state of mind, that is view of
In the context of an issue of self defence…. Then clearly a person would not be guilty of an assault unless the force used was excessive and in judging whether the force was excessive, the jury had to take account of the circumstances as the defendant believed them to be. In Martin (2002) the court of appeal was asked whether psychiatric evidence that caused the defendant to perceive much greater danger than the average person was relevant when a jury was deciding on the issue of reasonable force.
The court held that is was not revelant distinguishing the House of Lords decision in the leading provocation case of Smith (2000) Lord Wolf CJ justified this difference in approach There are policy reasons for distinguishing provocation from self defence… provocation does not provide a complete defence; it only reduces the offence from murder to manslaughter. There is also the undoubted fact that self defence is raised in a great many cases resulting from minor assaults and it would be wholly inappropriate to encourage medical disputes of that sort….
As a matter of principle we would reject the suggestion that the approach (in Smith) in relation to provocation should be applied directly to.. self defence,, the jury are entitled to take in to account,,,the physical characteristics of the defendant. However, we would not agree that it is appropriate,,,, in deciding whether excessive force has been used to take into account whether the defendant is suffering from some psychiatric condition.