In addition to the excellent advice above Clare, I would suggest that when you have obtained copies of the documentation your lender holds about this claim, you pay particular attention to the Mortgage Indemnity Guarantee (MIG) payout (assuming that there was a MIG policy taken out by your lender).(posted 7959 days ago)If the date of any MIG payout to the lender is more than six years ago then it may very well be the case that this portion of the lenders claim is now statute barred under the 1980 Limitations Act.
There is still legal uncertainty over this issue and forthcoming Appeal Court cases may or may not clarify matters so keep an eye on this site for updates in the next few weeks.