Section 3 Criminal Law Act 1967 states:
"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."
Additionally, there is a common law right to use reasonable force in self-defence or in the defence of another.
What is 'reasonable force' is a question for the jury. A court, in considering what is reasonable force will, in the words of the Criminal Law Revision Committee, "take into account all the circumstances, including in particular the nature and degree of force used, the seriousness of the evil to be prevented and the possibility of preventing it by other means".
This law takes into account the nature of the crisis leading to the use of force and that the person using force may have acted in the heat of the moment. Even the most reasonable man cannot be expected to judge with fine accuracy the minimum force required. In the words of one appeal judge, in a case where the amount of force used was in question, "In the circumstances one did not use jewellers' scales to measure reasonable force".
It is for the jury (ie ordinary people, like those who have contributed to this debate), having heard all the evidence, to decide whether the amount of force used by the accused was reasonable or not. In the Martin case, for example, they decided that, in the circumstances, the amount of force used by the defendant was not reasonable. In other, (less well publicised) cases, juries have decided that using fatal force on a burglar was reasonable. It all depends on the facts of the case, which, IMHO, is exactly how it should be.