Hello Paul, I'm Keith and happy to help you with your question.
Just Answer rules preclude me from recommending any practitioner, but you certainly should be represented by a trusted, local solicitor with a thorough knowledge of taxation matters. It won't come cheap, though, be warned, especially if counsel has to be briefed
HMRC do not like people mixing taxes like Income Tax, Capital Gains Tax and VAT. However some years ago, in an industrial dispute, some taxpayers did exactly that, offsetting one against the other and the courts supported them saying that it was all owed to the Government anyway, so hard cheese the Inland Revenue and HM Customs and Excise as they were then
Hope I have been able to help, albeit limited. Don't try to do this alone; too much money is at stake.