Please offer me some help with the CSA

16 years ago my husband had a one night stand which resulted with a child.

For the first 4 years he paid regular maintenance (although he was never allowed to development a relationship with said child) and got married. After only a couple of months of being married he lost his job and was placed on a nil assessment.Anyway to cut a long story short he split up with his wife and moved back to his hometown and shared a flat with a friend.He then got his life back on track and found a new job etc but no he didnt tell the csa and he didnt hear anything else for 10 years or so.

Anyway 18 mths ago a letter lands on our doorstep from csa demanding arrears payments and regular maintenance, we paying what they stated butalso now facing enforcement for arrears which wontbe paid within the 2 years which is thousands. we have seen our dp files which show that at the time in question (10 years ago) the csa were writing to a relatives address and no letters were sent to the address where my husband was living so we provided evidence they needed to prove my husband wasnt at the address they thought he was ( may i add that the assessment didnt include housing costs etc cus he was classed as a dependent living with a relative which is why arrears are high).

We hoped that this would persuade them to reassess the arrears taking into account the true circumstances but they responded by stating that they accept he wasnt living at the address the assessments were sent to but that they are correct in law as the address was given to them his ex wife who at the time had permission to speak on his behalf.

Does anyone know what we can do now? Can they really do this because we have proved he wasnt living there?