The organisers of this "immersive theatre" event took flak when it was announced. 

Critics claimed it would make light of the trauma suffered by victims of covert police operations.

A blog post from Blast Theory and Hydrocrack denied the charge and distanced their world premiere from the behaviour of undercover police who had formed deep relationships with the unsuspecting activists they targeted.

So what is it about? A flurry of text cryptic messages directs participants to a safe house where they are briefed by "police officers" who are clearly graduates of the Christopher Walken school of charm. Kitted out with false identities and back stories, the group head to a nearby pub which, it turns out, is the local boozer for every crackpot conspiracy theorist on Twitter. 

The job is to identify persons-of-interest (or POIs as the acronym-addled handlers want us to refer to them).

This is easier that it seems as the "extremists" drink nothing harder than orange juice so they don't forget who they are, while the middle-class festival goers are conspicuous by their nervous grip on a glass of booze and the pained expression EastEnders' Dr Legg used to wear when he wandered into the Queen Vic.

No spoilers here for those who are signed up for the month-long performances. All I'll say is that the "extremists" I encountered represented as big a threat to national security as the monsters regularly unmasked by Scooby Doo and the gang.