I am personally not a great fan of unions simply because of a long past experience (in another country) where I found my union more concerned about my employer than me...
The main union in the UK is http://www.bectu.org.uk/home - I have never been a member, but I know people that has been and is very happy with it. Bectu also have a very large document section including rates etc.
However, bottom-line though is that a union does not guarantee you fair practice, this has to be the responsibility of the individual. And as harsh as this comment is; you are guilty in accepting to do a 52 hour job "just to be the hero", when in fact you are putting your own and other peoples lives at risk - one of the industry's hidden killers, including that of innocent bystanders, are when workers/freelancers without enough rest decides to drive their own car home and fall asleep at the wheel... So learn to say "No", and negotiate your own fair terms, but don't rely on organising a bunch of hungry film makers.
I appreciate your help and your comment is not harsh, it is perfectly reasonable.
It was meant to be a job with normal hours which overran spectacularly due to a very inexperienced director, and is something I avoid these days.
Saying no is part of the equation, the other critical things we do need are insurance; crews often work completely uninsured and shoot overseas with vanilla travel insurance, if any at all, and the terms are voided because of the nature of work. Saying no would be a great option if the practice were not as prevalent as it is.
These issues do need a group of some sort to broker favourable group rates for the collective. There is currently only one private insurer who provides insurance for the production industry and it is based on individual rates.
Thanks for your input and the information it is very much appreciated!