I have the exact same problem as you, i worked a week and am owed quite a bit. The week after i constantly contacted them and they were rather fed up with me. I have been hanging on waiting for the post to arrive because thats what they keep promising. My problem is i didnt sign a contract, did you?
Also what i was thinking of doin was writing to them about taking them to the small claims court and that i will be instructing a solicitor at my earliest convience, could they possibly provide the name and address of their solicitor to start proceedings please. Thats what i hope will scare them and make them move!
Good luck and i will stay in contact if you wish to see how we get on!
Thanks Louisa