I bailed on the 820 in the end. It was only running 3-5 hours in cold weather and 7-8 in nicer temps and I was coming up on the Extreme X with a planned 11 hours in the Lakes. Go figure.
Pre-ordered the 530 which was only due mid-May at Wiggle - for a race on 25 May. Worried was an understatement. But magic happened and it arrived early May and all ready for the race.
So after 7.5 hours on pottering around the hills in the Lakes it went from 100% to 70%. Believe the hype, this baby can actually do 20 hours. Very impressed with it so far, massive step up from the 820 in all areas. Much faster processor as well.
Reason not to get the 830 was a) I rarely navigate on the fly, and the 530 can do turn-by-turn with GPX anyway, b) the 530 has REAL BUTTONS and not touch screen so works all gloved up, and c) price is rather a lot lower.