When you buy a house the selling solicitor pays off the sellers mortgage from the sale proceeds. An undertaking is given by the sellers solicitor to the buyers solicitor to pay off the mortgage and hand over a document to remove it from the registry. This document comes from the mortgage lender. Sometimes the banks are slow with their paperwork, sometimes they loose things, but these are not irreplaceable documents. 4 years is however a long time. Your solicitor may have the document and not have gotton around to sending it to the land registry or the sellers solicitor may have been slow to send it on, o Either way as long as the mortgage has in been paid off there's nothing for you to worry about, it would take 2 very incompetant solicitors to sell and buy a house without the mortgage being paid- not to mention a seller rich enough to not notice making the monthly repayment after they had sold the house. It is not going to put off any buyers, as long as it's dealt with before you close the sale they have nothing to worry about either. When you meet your solicitor regarding the upcomming sale ask for an explaination of what has happened and get an assurance that it will be dealt with.