Tax Credit - Fraudulence?

I've been paying back an overpayment to the Tax Credit people with only two more installments to complete. Yesterday I received a threatening legal letter from the twats, oblivious to my current mandate, and asking for an extra �150 over what I owe. I have kept a record of all paperwork, and can substantiate all correspondence, agreed payment records etc. I would like to ask anyone in the legal business, can I take them to court for fraudulent claims? And if so, what are the success rates?