Eventually after trying several other recovery options I had to resort to a full factory reset. This has worked a treat though restoring all the outstanding updates and various other tasks means its taken most of a day. I now have a working Surface. I haven't restored all my apps as yet as I suspect it was one of those that was causing the problem.
For future reference I'm adding the instructions on how to do this
I found this on another forum:
The only way to kick off a RESET without signing into Windows is to interrupt windows startup a couple times to get Windows Automatic Repair to start:
1. Turn off the device by holding power for 6-10 seconds
2. Tap the power button to turn it back on.
3. Once the surface logo appears, wait for circling dots to appear and then hold power for 6-10 seconds to turn device off again.
4. Repeat steps 2&3 until the surface logo appears with words underneath saying “Preparing Automatic Repair”
5. Recovery mode will start.
6. At the Bitlocker screen, tap “skip this drive”
7. Tap advanced options
8. Tap Troubleshoot
9. Tap Reset…
10. .. tap Just remove my files.
11. Reset process will take approx. 20-30 minutes – all data will be removed from the device.
After the Reset process completes, the OOBE/start up wizard should start and the customer can complete setup of their Surface device.