I should have realised this sooner, location services are built into W10 and other operating systems. It certainly isn't using IP address for location as the location given is exact.
I wonder if you have location services turned off and your PC is using an old cached location.
You can clear location history in settings>privacy >location
I'm still not sure how GPS and wifi location are somehow transferred to Ethernet connected devices but they are. I have just fired up an Ethernet connected desktop with no wifi running Ubuntu and that picked up my exact location (a very small village) immediately.
I completely forgot about the default location in Windows 10. Have changed this to my new home address, and it appears to have helped - certainly in firefox, and in an incognito chrome window. Normal chrome windows still show my old address in Portsmouth, so not sure how to fix that!