"11(b) Self-Defence/defence of another (i) PS2 would not be guilty of murder or manslaughter, attempted murder (or the alternative of wounding with intent to do really serious harm … if he assaulted and killed SW1 but at the time had acted, or might have acted in reasonable self-defence/defence of another. (ii) This defence is available to an accused when the explanation for the use of force is that he believed it was necessary to use force to protect himself and/or another from an attack that was either ongoing or imminent. Plainly the words ongoing and imminent are very important. An ongoing attack is a physical attack that has actually begun. An imminent attack is one which is likely to occur at any moment. I direct you that in this case you must consider the very moments surrounding the event. (iii) There are essentially two parts to the defence or two questions that you would have to answer: 1. Did PS2 believe or may he have believed that it was necessary for him to have to use force to defend himself from an ongoing attack or an imminent attack on him and/or another (consider his state of mind). If yes, then: 2. Was, or might, the amount of force he used be reasonable in the circumstances as you find them to be. (iv) Once raised it is for the prosecution to make you sure that PS2 did not act in self-defence/defence of another. PS2 does not have to prove that he did act in self-defence/defence of another for it to be available."
"Whilst PS2 must be judged on the basis of the facts as he believed them to be as to whether the use of force was necessary, it will not be sufficient to excuse him, that he honestly believed that the force used was reasonable, as the force used must actually have been reasonable on your assessment. As to what is reasonable force for the purposes of protecting himself or another (here his friends), in a moment of emergency when individual action is necessary to prevent some imminent attack, it may be legitimate for PS2 to use force of his own initiative but when police officers, if called upon, would be in a position to do whatever is necessary, the citizen must leave the use of force to them. If and only if, the law enforcement authorities will not assist (perhaps for operational reasons), PS2 as a citizen whose own person (or friends) are under threat, may take reasonable steps to protect himself or them; but the right of the citizen to use force is, as you will understand, much more closely circumscribed under such circumstances. You will bring your judgement to bear on this aspect of the case. If PS2 did use force under such circumstances, bearing these things in mind, the force must be reasonable on your careful assessment of it."
"In a moment of emergency, when individual action is necessary to prevent some imminent crime or to apprehend an escaping criminal, it may be legitimate, praiseworthy even, for the citizen to use force on his own initiative. For when law enforcement officers, if called upon, would be in a position to do whatever is necessary, the citizen must leave the use of force to them."