The basic principles of self-defence are set out in Palmer v R, [1971] AC 814; approved in R v McInnes, 55 Cr App R 551:
"It is both good law and good sense that a man who is attacked may defend himself. It is both good law and good sense that he may do, but only do, what is reasonably necessary."
The common law approach as expressed in Palmer v R is also relevant to the application of section 3 Criminal Law Act 1967:
"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."
Section 3 applies to the prevention of crime and effecting, or assisting in, the lawful arrest of offenders and suspected offenders. There is an obvious overlap between self-defence and section 3. However, section 3 only applies to crime and not to civil matters. So, for instance, it cannot afford a defence in repelling trespassers by force, unless the trespassers are involved in some form of criminal conduct.