Ok, I managed to fix the problem.
First I went back to a backup and tried the upgrade again, the same thing happened and I still didn't have WiFi.
Then I did a clean install using an ISO I downloaded, everything worked as expected although it wasn't activated, spent 90 minutes on the phone on hold to Microsoft to get it activated when eventually someone answered and put the phone down pretty much straight away. (Great service)
In the end I decided to download a Surface 3 recovery image from the Microsoft website, and followed the instructions to apply the image to my system. I had to run through 100+ windows updates then it allowed me to do the upgrade to Windows 10 again.
This time the update finished and happy times, I have WiFi!
I think the problem is some underlying OS corruption interferes with the upgrade, best bet is to just start from scratch.