Lucy Parker’s film is about a nasty, dirty, century-old little secret at the heart of the British state: industrial surveillance. It is about the Blacklist Support Group, formed 10 years ago when it was revealed that construction firms such as Balfour Beatty and Carillion had been submitting names of supposed troublemakers among the workforce (people who were sometimes doing nothing more than demanding back pay and union representation) to a central snoopers’ body they were bankrolling, blandly called the Consulting Association.

This grew out of another body called the Economic League, formed in 1919, and its mission was to circulate – and continuously update and expand – a blacklist of workers who would then mysteriously find themselves unable to get jobs. Keen to justify their meal ticket, the beady-eyed blacklisters would look for more and more people whose working lives they could ruin: people who sometimes were doing nothing more than writing to the local press on leftist issues.

Then the Data Protection Act meant that their little list was illegal and the Consulting Association found itself in court in 2009. But Parker’s film shows how questions are still unanswered, mainly the group’s interest in environmental activists and the extent to which the police and special branch offered their own help to the blacklisters. There is also the question of how surveillance is morphing and evolving with infiltration and undercover work: police officers seducing and even forming relationships with women in the environmental movement – sexual opportunities being apparently a perk of the job.

Parker’s film is not perfect – it is a little unfocused, taking on a lot of disparate but related issues in its brief 75-minute running time. I would have liked to see more on the grim history of the Economic League, perhaps more talking-head contributions from historians and trade unionists, although Parker has plenty of sparky and pertinent material from the campaigners themselves. It’s an eye-opener.

• Solidarity is released in the UK on 4 October.