I've been using a 310XT for a couple of years and for some unknown reason I had problems over xmas with the ANT+ trying to do the upload and then reporting a failed transfer, possibly like you say, Nigel.
One of my suspicions was that a corrupted data file on the watch was somehow bothering it. Plus I¨'ve had problems in the past when the memory filled up with runs on the 310 and with a corrupt file on a 110. So since all but the last run had been uploaded anyway, I first deleted all but the last month's runs from the watch. That didn't help. So I told it to delete all. That left two runs in the History that stubbornly would not disappear. ANT+ still reported an error when it attempted to upload every few minutes.
I then googled factory reset and found instructions on garmin. (Note that this is different and more comprehensive than soft reset.) It solved the problem. Three or four runs since have stored fine and uploaded fine. The mystery undeletable runs of course disappeared with the factory reset. I still love my watch 
The drawback is that you lose all user settings. I advise you to make a note of your heart rate or pace zones, data-field set-up for each screen and any custom autopause, footpod calibrations and the like, so you can set it up again as you are used to. Having said that I started from scratch and am happy simply with three data-screens now: one with time, pace, %HR, clock time, the second with 4 fields for interval sessions, the third with 1 field of %HR.
PS here's the Garmin factory reset, but view it as the nuclear option.
Edited: 10/01/2013 at 09:54
