@Greenroofer, they all show similar speeds, but I didn't consider videoing it. Darn... it took ages to fit all the rubber bands too! Maybe one for a follow-up?
The Edge 510 is conspicuously much better than my Edge 500 when there's a lot of tree / building cover and I'm on foot. Both are OK at the higher speeds of cycling, perhaps because the algorithm has a better chance of getting it right when it knows you can't be swerving that much at X mph?
@davemorrison, I think you may just be seeing an accessory for the Vector pedals, which are sadly more like £1300...
When I bought my PowerTap it was on mighty sale but even so I thought it was a bit ridiculous. Never had a moment's regret though.