Between the Lines S02E05 - Some Must Watch...

Special Branch, watching a T.A. centre, witness a potential raid on its weapons store; armed officers shoot a 68-year-old workman by mistake. The intruders claim to be working for MI5 and refuse to assist Clark's investigation. It transpires that MI5 were bugging the centre to entrap a junior officer and a Special Branch Inspector suspected of selling arms to terrorists. After a number of machinations, the official position turns out to be a whitewash, and Clark learns that the tentacles of the security services reach into his activities.