Many many thanks to everyone for all the advice offered. I very much appreciate the support.
I have finally found success!!!
I just need some clear skies and a good target.
I though it would be worth posting with my findings.
After update to StellarMate 1.4.0 I still had no success with the usb-serial connection. In addition, the update seems to have broken the web manager.
So I'm back to 1.3.5 again which is working well.
I have a new USB-Serial cable, this time with a genuine FTDI chip, and this has worked "out of the box". Since it has a different vendor/product ID from my GPS then I don't even have to use the Serial assistant or any UDEV rules.
It would seem that the CP210x chip does not play well with StellarMate.