It may not be much help, but, if I were buying a house with Grade 2 listing, and renovations carried out on it, I would firstly consult my solicitor, ask there advice, as they more than likely know the avenues that should have been taken before any work had been carried out. Then I would contact the local Buildings Regulation Council to 1:- see what work can be undertaken for a Grade 2 listed building, 2:- see if the work that has been carried out had to be notified If it had to be notified, and was not, from a grey part in my memory I believe that YOU will be liable for any works that need to carried out. Hope it hasnt put you off but that is what I would do if I were in your position.
Good Luck!