have a search for a SiRF III Bluetooth GPS Reciever on google (its currently £29.95)
I bought one of these a while ago and its great. It uses the SiRFStar III chipset which i think some of the TomTom's use
I use it with my old XDA II and havn't had a problem yet. it's managed to get me home from wales to notts with no problems
there wasnt any proper destructions with it, but was easy enough to set up. just paired it, set my XDA's bluetooth to accept inbound conenctions on a com port, then set the reciever up in Tom Tom nav 5 to use that com port