The Public Order Act 1986 s.2 states:
1) Where 3 or more persons who are present together use or threaten unlawful violence and the conduct of them (taken together) is such as would cause a person of reasonable firmness present at the scene to fear for their personal safety, each of the persons using or threatening unlawful violence is guilty of violent disorder.
2) It is immaterial whether or not the 3 or more use or threaten unlawful violence simultaneously.
3) No person of reasonable firmness need actually be, or be likely to be, present at the scene.
4) Violent disorder may be committed in private as well as in public places.