Thanks, both. Perhaps I should clarify one aspect of terminology, which I suspect may be a specifically British usage.
Over here, a "gangmaster" is a legal term, referring to anyone who employs people to do work which he or she is being paid by a third party to arrange. So rather than sourcing, interviewing and hiring all the seasonal workers needed to bring in the harvest, or all the unskilled labourers needed on particular phases of a construction project, the farmer/developer will contract with a gangmaster, who will handle all that and simply turn up, on the day, with all the necessary labourers, and then at the end of the day, drive them back home and pay them himself out of the fee he's received for arranging their services.
Gangmasters are supposed to be licenced, and most of them are and perfectly above board. But some aren't as scrupulous, and the system is certainly open to the most dreadful abuses, as we've seen.
It's not just undocumented immigrants brought here by people-traffickers the rogue gangmasters exploit, although they're obviously particularly vulnerable to this sort of abuse, but also people from elsewhere in the EU who maybe don't speak much English (and have family back home for whose welfare they fear if they upset the gangmaster) and also, it's become all too clear, vulnerable Brits.
There's been a whole series of prosecutions here involving (typically) whole families who have assembled gangs of people whom they have effectively enslaved by approaching vulnerable and homeless people living on the streets with offers of board, lodging and pocket money and then kept them on their isolated farms, living in squalid conditions, unpaid, malnourished, and subject to regular beatings if they step out of line, where they use them as forced labour either for themselves or for others.
Learn here how the Gangmasters and Labour Abuse Authority (GLAA) protects vulnerable and exploited workers.
www.gla.gov.uk