another person if:
* the person is in the act of committing an offence under any Act or statutory instrument, or
* the person has just committed any such offence, or
* the person has committed a serious indictable offence for which the person has not been tried.

Section 231 of the Act allows the use of such force as is "reasonably necessary to make the arrest or to prevent the escape of the person after arrest". A person who arrests another person