I used to be a regular customer (and an appreciative customer) of Britannia Music until it folded last year - yesterday I received a letter from a credit agency (intrum justitia) claiming I owed them (as administrators) some unpaid money (> £30) supplying an 0870 number as a contact! Sifting though my orders I find the exact amount ordered (and paid for) back from March 2007. A web search appears to give lots of other folk with similar letters - the letter was quite threatening - if you receive one, do check that it is not for an amount that you have paid!
Either Britannia music's credit systems were in a mess when they folded, or the credit agency has a similar problem, or someone is seeing how much they can recoup by threats.
Fortunately I have the emails from Britannia recording my payment!
Later - as this posting is fairly high up the google hit list for likely search terms or other letter receivers, let me also recommend this thread and this phone number 0151 4732626 quoted in the thread, as an alternative to the 0870 rip-off number.