In reply to Dave B:
That was a very good idea. When I make those visible, I see this:
The whole Garmin drive has a capacity of 20.7 MB ( and mine has 53 KB free, straight after a reset - and so, if that is true, would be filled by a single run, which would generate a 100 Kish .fit file) The Garmin folder is about 3.6 MB. That hidden .Trashes folder has a BIN subfolder containing 170 .fit files, or 17 MB worth. It's everything I have done since last June. There are also 4 TempFit folders in there.
Why that .TRASHES is there and why it is hidden I don't know. But this must be the problem. When I delete all the .fit files, I now have 17 MB free on the watch!
Let's see if that has done the trick.
Thanks all for your help.
Post edited at 05:24