Any ideas?

If the child is no longer living with the parent then that parent should not be claiming child benefit - it's fraud. Child benefit and therefore CSA are payable until the child is 19 if they are in certain education, or doing an apprenticeship.

We had the same situation with my husband's child. Proving that the child is living at another address is another thing entirely. Also I do believe that the adult the 'child' is living with is entitled to claim child benefit and then open a CSA (CMS) case in their own right.

In our case for the sake of a continued relationship with the child we decided not to take it further and just 'bite the bullet' and continued to pay until the child was 19 - even when we told that child that we would happily make payments to support them directly rather than through CSA via the PWC, the child still tried to make out that they were living with the NRP when we knew full well they were lying. To stop the child and sibling having to continue to lie, and to stop them trying to avoid us in case they let something slip we decided to ignore it and continue to pay rather than take it further, as much as it really grated on us.