Question Author
I was hoping it was a simple setting that was wrong on the desktop, that someone could point out to me. The desktop is newer than the router and had a re-installation a few weeks ago, albeit with a wired internet connection at the time, and uses the same applications as the laptop. I can't remember when I first noticed the problem as the only task which requires an accurate position is finding the starting point when generating a route for my TomTom, which I usually do on the laptop anyway. Other tasks, eg storefinders, either ask for a postcode, or (National Trust etc) are happy with a general location for recommending places to visit.
It's more of a puzzle than a problem, so not worth a lot of effort to sort it but it would be nice to know the answer.