CSA arrears advice please

My partner lost his job last year, he has a small pension. He last paid his ex aug 08 the csa told him he had nil payments to make from his pension then they said he had been given the wrong info & he had to pay 20% which clearly stated on the csa website this was wrong.

He started work april 09 & told them straight away sent copy payslips p60 from his pension. Now they are chasing him for arrears saying his pension is £10,000 more than he gets per year & that the last payment he made to his ex was feb 08 not aug 08. he has sent all his bank statements clearly showing the payments but they say its not enough.

They have lost all the info he sent them. Now they have said they know he has a credit card & want him to pay the arrears on that.

We really don’t no what to do as they dont seem to want to sort it out.