What sometimes happens with debts, whether they be shortfalls, loans, credit cards or whatever, is that the original lender will still own the debt, but give responsiblity for chasing it to another company. That company will in turn employ tracing agents and other collection companies to find you and start making demands for payment.(posted 8237 days ago)I assume that the letter from DMS refers to the A&L mortgage. If so, then I would suggest that you write back and ask for more information about the debt as this is the first that you've heard you owe money. Also ask for a statement of how the debt has been calculated and supporting documentation. This should send DMS scurrying back to whoever instructed them.
In most cases, these collection agencies are just told to collect X many pounds from a person and if the person doesn't pay threaten court action, bankruptcy etc etc. These letters are usually enough to make people pay up, but if the worm turns and starts asking awkward questions, then the collector has no choice but to refer it back to the original lender.
You might find that if you write back to DMS then you'll get a response saying that your queries have been passed to A&L and that will be it. Then in a few months you'll get a letter from another collection company demanding the £20k all over again.