Hi All
I know you have returned your Ambicom Bt112 antenna Barney, I have one too and this is how you get it to work with TomTom incase anyone else is interested.
The problem is, you need to be able to select the OUTBOUND bluetooth serial port in the tomtom gps setup, which it doesnt let you do in the normal version. However there is a patch that Tomtom have released that will fix this issue so the Ambicom and other third party bluetooth antennas will work.
Get it from
http://www.tomtom.com/updates/gpsupg307.exe
Once this is installed on your PDA you can now select the outbound serial port in the GPS setup and providing you have selected the correct protocol (ie NMEA-0183v2 Bluetooth) in the pull down menu above it should work fine.