Hi Wayne - I kind see how a buyer would be willing to complete on their purchase until the mortgage was vacated
If it was vacated in 1980 then the lender would either have signed off on the deed itself, known as an endorsed receipt, or provided a Form 53, the prescribed form at the time.
You have attached a scanned image but only of one part of the deed but I assume your conveyancer has already checked the whole deed for you.
Midland Bank is of course no more but their interests now come under HSBC as you state
The mortgage is presumably still protected by way of a land charge so an application to cancel same should be considered -
The land charges registers and agricultural credits register: registration, official search, office copy and cancellation (PG63) - GOV.UK
I would recommend checking the entire mortgage deed and other paperwork again. And referring your conveyancer to PG 62 as appropriate although they may have already considered same and an application to the Land Charges Department