what does it do to your average time and stuff (not that I'm too bothered, just interested to see)?
If you're uploading to Strava, all your averages are calculated on moving time rather than elapsed time, so you can rest assured that you won't be made to look even slower than you already are.
Also just looking at this but it always cocks up the elevation afterwards so you'll see (maybe) that it'll say on the way in it's 1-5ft but on the way home it's like -60-65ft. Not sure what's up with that..
I used to have this problem until I realised that...
Depends on the Garmin model, but on the units with a barometric sensor if there is a change in air pressure during the day (quite likely) it will make a difference to the elevation recorded. If the weather got worse as a result of falling air pressure the garmin will record a leap in climbing/ascent.
...this was the answer. It took me a while to cotton on - I didn't even realise my 510 had a barometric sensor until very recently. Wet weather can interfere with the sensor too but it's usually down to turning the device off when you stop and turning it on again later, by which time atmospheric conditions have changed.
The 510 has more than enough battery capacity to leave it running for the duration of a 200km audax without turning it off at controls, so now I don't bother and just leave it outside on the bike. I've even turned off the auto-pause. It's fine. I have a battery pack to top it up for longer rides.