Presumably you did not get building regulations approval either. I don't think you can get this now because it involves inspecting the work while it is being done (depth of foundations, suitability of drainage etc.). You may be able to get an indemnity insurance for quite a modest cost which would cover the purchaser against the relevant risks.
You need to discuss this with the solicitor who will do the conveyancing for you. He should be able to advise you on the best course of action. Your estate agent should also probably be informed.