Thanks for your question. I hope I can assist you.
The planning department of the local authority will hold records of all planning documentation relating to the property.
Any enforcement action for lack of planning must be brought within 10 years of the breach. If there is no planning permission then the planning department is now time barred from bringing enforcement action. The conversion would have needed building regulations approval too.
The simplest / cheapest way to deal with this now is by way of indemnity insurance. Your solicitor can obtain lack of planning / building regulations approval indemnity insurance. You pay a one off premium. This does not make the conversion 'legal' but 99 times out of 100 it is acceptable to a prospective buyer.make sure the indemnity insurance covers mortgagees so that a buyer's lender is covered.
This should allow your sale to proceed.
I hope this helps.
Please let me know if I can assist further. Please come back to me if you have any further questions or queries in relation to my reply.