Bluetooth GPS BT3100 wont connect to MDA/TT3

[Geordiedefector]

HELP!! I have a new T Mobile MDA which replaced my bulky MDA3 and I'm trying to connect a BT3100 bluetooth GPS to it. COM5 seems the right outbound COM port when paired with the MDA but TT3.7 GPS just wont let me tick the COM5 box in the settings! That is to say when i tap on the box nothing happens...no tick appears......It did once (at the beginning but not since).
Any advice ?!?!??!?
OR am I doing this all wrong?! I didn't think I was a muppet but maybe I am?

We see you’re using an ad-blocker. We’re fine with that and won’t stop you visiting the site.
Have you considered making a donation towards website running costs?.
Or you could disable your ad-blocker for this site. We think you’ll find our adverts are not overbearing!