Before you go further hasnt your lender supplied a post repossession valuation by a qualified surveyour in your SARN + evidence of marketting of the property.(posted 7584 days ago)What does this valuation say - is it an independant company
also you only have 6 years (from the date you were informed of the amount it was sold for)to use this as a defence
+ if you bought in mid to late 80's and sold in early 90's you can more than likely forget it - too many repos and therefore the sales market fell through the floor, houses lost value month by month.
If it still looks as though your on to something a fee is payable to the lard registar for this kind of information. Contact them first using their website as guidance www.landregistry.gov.uk