Sounds to me as if their tactics are working Bev.(posted 8050 days ago)Hang in there, and keep doing what you are doing - they will eventually get p*ssed off and start making bigger mistakes in their letters to you - will give you more to work on.
You could accuse them of protracting the matter for a start (assuming you haven't done so already).
Why is it hanging on your credit file?
Stephen