This is a common law offence which means that it does not come under particular legislation. The law surrounding it is from case law rather than legislation by parliament.

If you look at what must be proved that is basically what the law is in relation to this charge.

There is then a lot of case law that explains this law in greater detail.

10 years imprisonment

1 That three or more people were present

2. That these people had a common purpose

3. That they executed, or started the execution, of this purpose

4. An intent to help one another by force if necessary against anone who opposed them

5. That they used force or violence in such a manner as to terrify at least one person of reasonable firmness

Factual dispute

Lack of intent