Self Defence although seems to imply that it can only be raised when the accused is under attack the defence can also be raised when defending another person or persons from harm.
Traditionally force used to protect another was only allowed if a relationship existed, for example, master and servant, parent and child. However, in today’s standards self defence is not restricted and it extends to helping others in the same way you would defend your self if under attack.
The law also allows intervention by a person on another’s behalf to prevent a breach of the peace. Common law (Judge made law) has often provided criminal and civil liability for individuals who prevent a breach of peace. However the steps taken to do must be necessary and reasonable. Things such as trespass, assault, dispersal, detention ad confiscation of property have been allowed and viewed as reasonably steps to suppress a breach of the peace.