The solution for me;
- I contacted the IT reseller, who in turn contacted the organisation which disposed of the Surface Book. The serial number was still in their Intune dashboard, so was removed.
- The Surface Book still asked for a work/school account. At the screen where this was being asked, I pressed Shift-F10 (you might need FN as well), to call up a command window.
- The I entered 'start ms-cxh:localonly'. The surface book then allowed me to set up a local admin account.
- From there I ran updates for Windows, which also updated various firmware items.
- After the updates and a reboot I could then add my personal account, and all is well.