Thanks for the messages. Yes I am right in the sticks so that could be it, I have to travel round the M25 at Christmas so i will see if that picks it up. Only other thing I can think of is if the sat nav disk is not the right one for for TMC. Not sure I can be bothered to do the 3 hour round trip to drive to Southampton to go back to the dealer so may pop into Marshals in Cambridge at Christmas and see if they have any ideas (by all reports they are a very friendly bunch).
I can't work out what version of sat nav software I have, can you get the display to tell you somewhere in the menus? Disk is 2005 - 2006, blue with lots of codes but no obvious version number.