Hi I did some more research and came across the following:
https://www.windowslatest.com/2019/09/25/microsoft-has-finally-resolved-surface-pro-battery-issues/
It was suggesting that Insiders have been getting some new firmware to sort the battery drain problems.
I did it slightly differently by joining the Slow Ring rather than the Release Preview option, but surprise surprise along with the Preview version came a list of 25 driver and firmware updates.
I am ready to test overnight and tomorrow to see if the battery drain when off problem is resolved and let you know.
For interest my new wesion is Windows 10 Insider Preview 19033.1 (vb_release) and so far I have not experienced any issues.
I will update with the result od testing.