OK I have recovered the letter and gotten back home. There's nothing untoward on the front of the letter, just my address, the postage paid stamp and the Special Delivery sticker.
On the back there is a return address for a solicitor...
It's the solicitor I used for the purchase of my flat late last year. What do they want?
IT'S GOOD NEWS!!!
We are currently undertaking an exercise to identify client account balances on apparently completed matters. We have a residual balance of £84.01 on your account.
As a result I enclose a cheque in the amount of the balance
Awww yeah, drinks are on me boys! Good things do happen to bad people.
Result!