I did try a master reset in the pub, based on a phone google, but I got a page full of firmware versions and stuff rather than the "do you really want to do this" message, so I expect I did something wrong. I'll try again.
I probably want a new GPS anyway - the release catch on the handlebar mount has been broken off for the last couple of years (at least), and the lanyard attachment point broke right at the beginning, so it's rather battered, having bounced off down the road at speed on several occasions.