Depends what you want to use if for, You posted on a MTB forum, so the assumption has to be that you will be taking the bike off road and riding hard. Have been out with rides on bikes upto the £800 range, and thngs have a habit of falling off and not lasting too long.

For the budget you are looking at, just pick the colour of the bike you like, they are all so similar, disregard full suspension, house brands, like Halfords Carrera & Boardman, or Evans Pinnacle will offer good value for money, also you can find good deals on previous years models, but you will be limited on sizes, 2015 models are just around the corner, so if you can hold out a month or so, the 2014 ranges will be discounted.