civilian was apprehended on a married patch acting suspiciously
You probably can't carry out a Citizen's Arrest for acting suspiciously. The only grounds are:
• Arrest for an "indictable offence" under the Police and Criminal Evidence Act 1984.
• Arrest of persons committing, or about to commit a Breach of the Peace under common law.
• Use of reasonable force to prevent crime or arrest offenders or persons unlawfully at large under the Criminal Law Act 1967.