Ian
I'm assuming you are trying this outdoors in the car, rather than indoors?
Mine takes anything from 30 seconds to 1 minute to find the GPS connection.
Open TomTom, select the GPS connection option (page 3 of the Change Settings screen), and then configure - that will let you see all the steps - as you say, COM1 is the sync connection, and I used that on my wired receiver and it works fine. The ony problem i have is that the connector from the GPS untit to the power cable is a loose fit, so I had to tape it up to stop it falling apart.
Just a few things to look at - hope you sort it cos it's a bugger when you've got a new toy that won't play
Speak to you soon
Regards
Martin