Hi Linwin,
I think you have been really unlucky in this instance and also think that your buyers were perhaps badly advised by an over cautious solicitor. This would not have been an issue for me as a buyer.
I would suggest posting your question on money saving expert, in forum for buying and selling houses. There are some great people on there with more expert opinions than mine.
However from what I know here are a few comments:
Building control can only enforce non compliance within 1 year. So if the conversion was in 1990 they wouldn't be enforcing any action.
Could you ask the for a letter stating that they have no interest in taking any action? This might calm future buyers.
I think if buyer was concerned about this, it would not be because of any possible action being taken, it would perhaps be concern about the standard of the work and whether everything was safe and done well. Building control wouldn't be able to tell you this and the regulations will have changed from 1990 anyway.
So if I had a concerned buyer, I would suggest that they got a RICS structural engineer to look over the property and then confirm it's all okay. So I guess this is something you could do in advance, to pre-empt any issues, and you could give the report to your solicitor to pass to theirs if need be but I honestly would wait, and suggest that this is what a new buyer did as you may be wasting your money getting someone in advance.
The buyer may ask you to cover the cost, or not, or to share it, but that's something you could decide when the time comes.
As a final idea, as it's a conversion, did any of your neighbours move in after you? Could you ask them if they had any issues similar to this and how they overcame them? Might be worth a conversation.
I hope this helps. We are going through a house sale and it's all just a horrible process. I hope you find a new, less nervous buyer very soon.