Proving when a house was built

Looking for help proving to a mortgage company when a house was built please. The valuer has vaguely put the property as '1965 approximately' and based on this the mortgage company are refusing to mortgage. The house is part of a a new town build and the town's own council Development Plan shows a timeline that the estate along with some others started to be built in 1975. This is the date I was told when I bought it in 1991 and no mortgage company has every queried it. The council planning office have an application for water and sewage to be laid down on the estate dated 1967. I have a document showing the property changing hands in 1981 and relates to the development company, new town corporation and the occupiers. This date and the document contents imply the build date would match to 1975. I have deed numbers on this documents for the house and the developer ownership of the whole build area. I have been advised by a retired solicitor to get the 'Proprietorship Record' but I can't see that anywhere. Can someone advise what forms I need and what I should be asking for? Thanks