It seems pretty clear: get Santander to formally confirm that you have (or had) good title to the car. Send that to MB and invite them to sod off.
That means you sold it free and clear to MB who sold it in similar state to new owner. Everybody is happy and nobody has lost out, apart from Satander who will have to recover the debt from the original defaulter.
Sound to me like you are dealing with a twerp at MB direct.
Unfortunately, this is not as straight forward as everyone seems to think. Santander will never send such letter (see below).
No, I am not a solicitor but I know the outcome.
1. Dealer sells car to client. Has outstanding finance, but shows a "clean" HPI (this HPI is normally printed before the Finance is taken out).
2. You drive home in your new car, happy as a daisy.
3. You either keep the car or sell it on (makes no difference).
4. Dealer who sold you the car goes into Liquidation.
5. You (or the new owner you sold the car to), are contacted by the finance company, in this scenario - Santander. They claim that they still have titel in the car and wants it back (or money). You (or new owner) protest of course, saying that you bought the car clean HPI etc etc etc. They then tell you that the seller had no legal right to sell the car, without asking them for permission first.
Problem here is of course that you never took out this agreement, the dealer did. But the dealer does not even trade anymore so Santander is going after the first buyer. Legally, they are 100% correct, they still do own that car, even if you sold it on and as you were the "first" buyer since the dealer (who does not exist anymore) you will get for it.
You could always fight this in court, but the chance of Santader loosing this is almost zero.
6. You contact a solicitor for help. Not much they can do except charge you a £1000 for sending a few letters to the former directors (I can help you finding these addresses) and then trying to convince the court (only the liquidated company's liquidator can ask a court for permission) that the director of the company who sold you the car has misrepresented the company under law so and so. This is the tricky bit.
The liquidator will always pay salaries, tax, vat etc first, then everybody else, including a finance company. So they go after the easy target first.
Of course, contacting Trading Standard is a very good bet. They are very helpful in my experience.
Someone else mentioned about "Section 75 of the Consumer Credit Act 1974". This is only valid if you pay a deposit on the car (or anything) above £100.01 and less then £30,000. So for example, you pay a deposit on a car at £500 using your credit card and then pay the balance on collection of the car using your debit card. You are protected. Any cash or cheque, you would not be covered. You can only claim this through your credit card company upto 6 months after the transaction.
I am happy to help as I said. Just send a PM.