For the record, Virgin had given me a Wifi dongle to get me over a delay in installation of my cable connection. Since the cable is now up and running fine I no longer needed the dongle and the separate Vingin Mobile account for the sim in the device is the one which was actually cancelled. I was basically unaware of this account as I never had to interract with the directly and unfortunately the cancellation notice did not include an account number to clue me in.
Sp, 5 stars to Virgin for giving me the dongle and account, less 1 star for scaring the crap out of me 🙂
And thanks again to Paul_Dn for sorting the issue out.