If the total costs to the lender (outstanding mortgage loan, arrears of interest, all selling costs etc.) total up to more than the selling price then there is a shortfall. If this is the case then yes, the lender can chase for it. In some cases for 12 years, but under a voluntary Council of Mortgage Lenders (CML) agreement in a lot of cases for 6 years. So they are well within the time limit.
They have to provide a detailed breakdown of what they are claiming, with the selling price and all the costs itemised. This needs to be checked carefully, and anything that looks dubious queried. If they went to Court with their claim, they would have to produce this detail so it is absurd of them to say they can't provide it. They should be told that, unless they provide it, nothing will be done to try to come to any payment arrangement because they have not produced any evidence that any money is owed. If they are a CML member and still won't act properly, complain to CML or FSA.
If money is owed then it should be possible to arrange an instalment payment arrangement based on the income and reasonable expenses of the debtor; alternatively if the debtor can borrow (or get from family) a proportion of the amount a full and final settlement offer can be made. If advice is needed on this, go to the local CAB or call CCCS.