I have just had a similar issue with tax credits. They said I didn't declare living with my partner when I had proof I had. Regardless of this they have been chasing me for £3500 for 3 years I sent 2 appeals to them but no one would listen to my argument. I then decided to go to citizens advice, they were amazing and spoke to hmrc for me, 6 months after I went to see them I received a letter saying I owed them nothing. If you can gather all the letters you can and take them to the cab, they will also ask tax credits to stop the debt collector letters until the matter is resolved