Answer: Enforcement action can still be taken. If the property was listed when the works were carried out in 1987, it is very likely to have required consent.
A surveyor experienced in listed buildings can advise if an application for retrospective listed building consent is likely to succeed.
If the surveyor feels that it may be granted, make an application so that the current situation where you have unauthorised works is rectified before you try to sell.
But if the surveyor considers it unlikely that you will be able to get retrospective consent, you can take out indemnity insurance, which can then be offered to a future buyer.
However, if that buyer wishes to carry out further works to the property that, again, are likely to require consent, then indemnity insurance may not be appropriate, as the cover may be affected in such circumstances.
In your case, building regulation and planning consent are not a concern.
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These answers can only be a very brief commentary on the issues raised and should not be relied on as legal advice. No liability is accepted for such reliance. If you have similar issues, you should obtain advice from a solicitor.