retrospective P. P. but no building regs

[Makitisi]

ANY ADVICE PLEASE????
In 2001 I bought a converted garage which was already being used as a residential studio and managed to get retrospective planning permission for change of use granted in 2003.
The conversion was done without permission or building control approval some time prior to 2001 - I have no idea when - possibly pre 1985,
- and though it has been dry-lined the insulation behind the plasterboard and under the nice wooden floor is only 1 inch thick so wouldn't meet 2003 regs.
I took out a mortgage on it with indemnity insurance without problem. I arranged for a site visit by the building control officer who came but then said that work done pre -1985 was beyond building control's regularisation.
I am now selling it and the buyer is asking for a certificate of completion from building control.
I rang the council and spoke to the current B. C. officer said all sorts of work may need doing including insulation, ventilation, double glazing, DPC, etc
The building is fine - well insulated, warm, dry and sound but will probably not pass current building regs. on any of these, and may need a huge amount of work done, really just for some paperwork.
Although change of use was granted in 2003 the building has been unofficially used for residential purposes for much longer than this.
What happens if i refuse to do all the work? Does Building Control have any authority to enforce after this length of time?
Can I instead just apply for a certificate of completion for the work I had done at the time I applied for P. P. in 2003 - i.e. the blocking up of a single door and partitioning of a bedroom?

[Richard Webster]

Buyer might be happy with a letter from Council saying because work done 1985ish they have no intention of taking any enforcement action, if you could get them to say that in writing.

In the real world thousands of people are daily buying older properties with all kinds of features in their original construction that wouldn't comply with modern regulations so it should all be seen in that context.