You can apply for retrospective approval (Regularisation Certificate) with the local authority building control, private inspectors cannot do this, however you will need to satisfy them it complies with the regs applicable at the time the work was carried out, this will involve exposing the work/carrying out any necessary remedial work, they may also ask for structural calculations although I personally wouldn't based on your info providing it is a uniformly distributed load.
As if that'll make any difference! I had an enquiry to do a completion inspection for an extension built in 1972 about 4 years ago, due to boundary changes it wasn't even in the same local authority at the time the work was carried out!
I never worked with the Building Regs 72, however I did work with the 76 regs which were very similar, essentially they were just updated to metric units with the omission of the regs for ashpits if I remember correctly
If you contact them, they will refuse to get involved because the work was done before 1986. Get them to confirm in writing that they won't consider it and send that to the mortgage co.
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