Paid holiday? Forget it, unless you're on that agency's books for ages and clock up enough hours.
You ought to get paid time off as soon as you've been working in the same place for a week or so: for every £100 you are paid for working regular shifts, you should get an extra £12.10 in respect of paid time off.
(From 1 April your entitlement went up to 28 days per year, following a European Court of Justice ruling against the UK government yonks ago.)
Are agencies cheating people and breaking the law, then?
