Nichola
We don't deal with Tax Credits on the forum so I am limited on the information I can supply.
It sounds, despite your previous declarations of having a CA claim in payment, that this has not been registered by HMRC and as a result they may be thinking that you may have been overpaid your TC's.
You don't mention what action may follow on from this letter, does it say?
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems