He was a burly, funny scouser called Mark Cassidy. His girlfriend – a secondary school teacher he shared a flat with for four years – believed they were almost “man and wife”. Then, in 2000, as the couple were discussing plans for the future, Cassidy suddenly vanished, never to be seen again.

An investigation by the Guardian has established that his real name is Mark Jenner. He was an undercover police officer in the Metropolitan police’s special demonstration squad (SDS), one of two units that specialised in infiltrating protest groups.