Although I understand the comments refering people to D-W, tbh that site is dead these days and a lot of the people who used to post regularly on there are on SDH now. So if you filter the crap, (which is obvious as they usually start their replies "I've never had one, but I've heard..."!!), there is a lot of sound advice on there.
Also those figures quoted above aren't the full facts as D-W had a total site re-build last year and everyone had to re-register so the count No's went back to zero. If it were total for the whole period D-W has been running it would muller the SDH figures.
There are quite a few 'mini DH bikes' around these days too, like the Commencal Supreme range, which are plenty man enough for 95% of UK tracks and also enable you to pedal up them too, which is pretty difficult on a full DH bike.
If you Def want a full DH bike, for £850 I'd look at the following;-
1) Santa Cruz V10 (ver 2) - Rolls Royce of DH bikes in the mid to late 00's. Need to check the back end bearings are 'tight' though and if not, negotiate discount to buy a new kit that is ~ £100 from Stiff.
2) Giant Glory - bomb proof DH bike,
3) Orange 222/ 223 / 224 - simple and cheap to upgrade.