Brian
It has worked, I now have a fully working Surface with all devices operating as normal. I believe that Bitlocker is now broken, but I hate Bitlocker with a vengeance. This episode actually made me ill.
4 Microsoft support staff were no help whatsoever. They would just send me the "boot from USB" instructions & close the case, without checking if it actually worked. Truly awful support for one of the most expensive laptops in the world, broken by their
own updates.
To fix this you need access to a second computer. You will lose all your software & data on your USB drive & your SurfaceBook2.
Download the media creation tool from here: https://www.microsoft.com/en-us/software-download/windows10
Put a 16gb+ USB drive into your spare computer. (Using USB3 will save a lot of time)
Run the Media Creation tool to the USB drive. (This will download Windows 10 & take quiet some time)
Take the completed USB drive & insert to the Surface Book.
Turn on the Surface & choose "Skip this disk" at the Bitlocker screen.
Go to the Troubleshooting option & choose Command prompt.
At the command prompt screen, type D: & press enter.
Type setup & press enter.
This will start the Windows 10 installation.
It will ask where to install it & present a list of 4 partitions. On my computer, partition 3 was the largest & the obvious location of my Windows installation.
Erase partition 3.
Create new partition & accept the default size. This will create a new partition 3.
Choose to install Windows 10 to partition 3. Windows 10 will run through the installation process & then the Cortana process to configure everything.
At this point you will have a working Windows with Internet access, but with basic drivers.
Download the Surfacebook drivers from here: https://www.microsoft.com/en-us/download/details.aspx?id=56261
I downloaded both.
Double click the downloaded file to install the first driver with the lowest file number in the name. You should reboot afterwards. Then double click the 2nd downloaded file to install the next drivers, then reboot afterwards.
Verify your user account details as normal.
Result will be a fully working vanilla laptop. All function keys working, drivers all correct.
Reinstall your software & restore your data backups. Remember that if you have Office 365, you will need to remove the installed computer from your account, before installing again.
Total time is about 2 hours work (I like to get things perfect). Plus 2 hours for downloading Windows 10 & the drivers.