Thanks for the advice. I was in touch with them this morning and have to wait for an adapter so I can backup the iPad and reset it it to factory settings and start again. I told them if that didn't work, I would be seeking a replacement and they said that would be the next step.
I advised that I'd turned off pretty well anything that took up power, apps in the background, notifications, brightness etc etc. But it still lost 10% overnight last night when off.
I've also drained the battery as someone suggested in this thread, and am now re-charging to see if that helps as I have to wait a week for the USB-A to USB-C adapter to arrive. My iMac is old and although I got a new hard drive and upgraded RAM and it runs beautifully, it doesn't have the right port for either my iPhonen or iPad! Technology huh?
Thanks for your help. I'll come back with the outcome in due course.