As a rule, under a £1000 for a full suspension bike will mean that the suspension & major components are not of the highest order. I'd recommend that if you want to buy new, stick to a hardtail. Go to your local newsie or WHSmith & buy
What Mountain Bike. There's a section to the rear of the mag that is full of bike reviews at various price points. The Halfords range is generally more toward the "road bike that looks like a mountain bike" until you look at the Boardman range. They are good buys for the price. Take your time going through the magazine, but don't treat anything as gospel.
I think the best advice I can give is this: Go find your LBS (Local Bike Shop). Call in, ask questions, drink their tea, ask more questions. If they are friendly & helpful, you're onto a winner. It's all very well picking up a second hand sh*tter from fleabay, or a brand new bargain from t'interweb, but your LBS will look after you & your bike during ownership. They will stay late to fix your knackered derailleur on a Friday night cos you're off to Dalby Forest Saturday at sparrowfart. They take the time to go the extra mile (usually) & look after their customers. That extra £100 you paid for your £2k bike will be paid back in spades when they see you're a loyal customer. My LBS is brilliant, and so helpful it defies belief. Don't forget that your bike needs to fit you properly. This may involve changing contact points such as pedals, seat, stem, grips & so on. Your LBS will do this. The web superstore won't care less once your bike's delivered.
If you find your LBS sucks, or you don't have one, go web shopping. Otherwise, it's just like finding a place you know to look after your GTI....
If you want more advice, pm me