You have two choices: you can either ignore the letters as Money Advice centres advise. Or you can use the experiences of the various people who have contributed to this site and go through the procedures outlined in the Do's & Don'ts pages (which is in the Repossession section).(posted 8633 days ago)You should read this site thoroughly before you decide what to do.
Naturally, I would recommend that you adopt the approach outlined ni this site.
There is one caveat: if you think your lender sold the property at a fair price and did not mis-sell or misrepresent any aspect of the mortgage, then it is hard to argue that the lender has done anythnig wrong. In which case you should make an offer to settle. But you should read this site thoroughly first.
Lee