What Mountain Bike magazine recommends the Carerra Fury at £550 rrp and the Decathlon Rockrider at £500 - these they rate the highest in their 'Value' price bracket. Bikes like these are coming on leaps and bounds and really catching up with some of the big brands.
I'd personally suggest you find any friends or friends dads who mountain bike and seek their advice - they might even be able to steer you towards something good second hand. Don't forget to budget for a helmet, clothing, lights and bike security as well. Oh - learning how to fix it yourself should be on your list of things to do.