FranP
1. Are they still living? Can they remember their solicitor or bank at the time. Either may still have them. Did they have a mortgage? Lender would have asked for them, and not returned them on completion of mortgage, but they will be able to provide proof of ownership
2. Presumably you paid cash and no deeds then? But did you register? Check with Land registry, it is just possible that it was registered at their purchase, depending on where it is.
I suspect that the OP's purchase of the property may have been an agreement between relatives and not done through a solicitor. In which case it may not have been registered with the Land registry. They may have to apply for possessory title.
I think FranP's questions are worth following up.
All property sales have had to be registered with the LR since, IIRC, 1986. We bought a property in that year and never had any deeds, not even when the mortgage was paid off.
Our current house is over 300 years old and though we do have some deeds for it, dating back to the 1920s, I'm not altogether sure why we do as it was purchased in 1994.
The LR only has some deeds (they don't have ours for a start 😀) so they may be no help at all.
Deeds may appear to be useless now but they can be a great resource for local historians. I'd suggest that if anyone has unwanted deeds they pass them to their local authority's Records Office. Someone might be interested in them at some time in the future.