Bluetooth mouse on dual boot Win7/Win10 system

This is a new XPS13 9343. I have set it up to dualboot windows 7 and 10. But I am having to re-pair my Dell travel mouse every time I switch from one OS to the other which is really annoying.

According to a number of posts elsewhere, usually concerned with dual boot Win/Ubuntu, I need to find the encryption key in Win 7. Then I would presumably have to get it into the Win 10 registry.

I have found what I understand to be the key entry in

HKey local machine/ current control set/ services/BTHPort/Parameters/keys.

But I cannot open it, even though my username is supposedly the administrator one. I have also no idea yet whether the key, if I am able to extract it, also exists in Win10, and whether i will be able to insert it.

Any ideas about a solution please?

EDIT: Actually, I've realised the simple answer to this. Stop faffing about with bluetooth and registry entries and just buy one of the wireless mice that uses a little dongle rather than bluetooth.  Got a Logitetch one from Ebay...Uses up a precious usb port but that can't be helped..