A person other than a constable may arrest without a warrant—
anyone who is in the act of committing an indictable offence;
anyone whom he has reasonable grounds for suspecting to be committing an indictable offence.
Where an indictable offence has been committed, a person other than a constable may arrest without a warrant—
anyone who is guilty of the offence;
anyone whom he has reasonable grounds for suspecting to be guilty of it.
But the power of summary arrest conferred by subsection (1) or (2) is exercisable only if—
the person making the arrest has reasonable grounds for believing that for any of the reasons mentioned in subsection (4) it is necessary to arrest the person in question; and
it appears to the person making the arrest that it is not reasonably practicable for a constable to make it instead.
The reasons are to prevent the person in question—
causing physical injury to himself or any other person;
suffering physical injury;
causing loss of or damage to property; or
making off before a constable can assume responsibility for him.
This section does not apply in relation to an offence under Part 3 or 3A of the Public Order Act 1986.