OK, sorted myself thanks.
This is what I finally did to set a file association for the .GPX
extension:
Control Panel > Default Programs > Associate a file type or protocol with a program > Select '.gpx' >
Change Program > Scroll to bottom (unintuitive IMO when looking at an alphabetised list) > More apps > Scroll AGAIN, beyond 'Look for an app in the Store' - a marketing motivated design? (that's the step I missed first time)) > Look for another app on this PC > Navigate to the specific executable program and open it etc.
A very klunky procedure compared to 15 year old XP method: Right click a sample file > Tools > Folder Options > Filetypes > Choose program from full list.
--
Terry, East Grinstead, UK