Matt,(posted 8452 days ago)read the board - you should have served your borrower/their solicitors with a SARN. They are legally obliged to give you these documents for a fixed sum (usually £10). There are example letters elsewhere on this board.
If you haven't served the SARN then do it now.
Your best defence is to say that the lender has not substantiated their claim. Say that you have not seen any of their figures and that you are concerned that they have not sold you property for the best price as they are legally obliged to do so. This is all documented in the Repossession and Debt Collection areas of the board...