Hurrahhh!!!!!!
Today I got a telephone call from Virgin saying that they were in receipt of my letter, (recorded delivery, sent on the 18th), and they would send me a MAC number once again. Judging by the time scale from letter to phone call Virgin may boast of the fastest broadband in the country but their internal office system resembles a forked stick.
This time I have a MAC No email and ALSO an office number to telephone if it does not arrive.
I'll keep you posted.