Sorry I don’t have relevant expertise nor experience to give you an authoritative answer.
I suggest first writing on your own - or with the assistance of CAB or PASS - making a clear concise request with your reasons. Include whatever medical evidence you have already - eg tribunal decision rejecting the diagnosis, or statement of another psychiatrist that the diagnosis was wholly unjustified. Make the least demand which will satisfy you. If the ICB does not make acceptable offer/suggestion then you can consider getting legal advice/assistance if the matter is that important to you.