Dear Karen
As per my above answer : "I have now performed a clean installof the Windows 10 May 2019 release and latest surface drivers. This was a complete wipe and reinstall - not an in-place upgrade.". ("Reset this PC" obviously wouldn't be this version of windows).
So it was indeed a clean install from a USB flash drive of the latest Windows 10 version from the Microsoft website.
I have disabled Fast Boot and tried various other tweaks (disable connected standby etc) - no change.
My Surface is still losing around 20% charge overnight.
Regards
Russ