Be aware that you can get 10% off at evans or halfords with a bc membership, and you can also buy vouchers for both of those at 7 or 10% discount through many employer discount schemes. Halfords have the 820 at 235 and the 520 at 180 pre discount, which is probably a better deal thank this.
I'd go for the new Garmin Edge 130 instead. It is a bit smaller and cheaper. And it has most of the features of the 520, except some of the navigation/mapping.
Ended up buying an Amazon Warehouse bundle this morning for £170. Had a stack of credit to use so it worked out to cost me £98. Considering I want it for data collection round my local area it will do the job nicely. Happy days. :D
I believe they are identical apart from screen size & one fewer row of leds.
520 battery life big issue. Read on Garmin forums https://forums.garmin.com/forum/into-sports/cycling/edge-520-aa/1286238-firmware-12-50-battery-drain-issues