In reply to richyfenn:
Agree, with small claims suggestion.
Unlikely that you will actually have to make a claim though. If they think that you are serious (and you know your rights) then they will normally fold.
As the commercial party, they are obliged to defend themselves at your local court, so just sending a representative (even if they are successful) would be more than it was worth.
Give them every opportunity to pay, with a clear statement of the facts, and defined deadlines. Keep copies of everything you send, and use recorded delivery etc.
There are lots of online guides about how to use the small claims process, and no doubt CAB can help. I've used it a couple of times before, and never actually had to go to court.