In terms of mountain bikes that's pretty low end.
But fear not, as £500-£1000 is a decent budget if you're canny. In particular, if you don't mind last years model many retailers will sell you bikes for a grand that were near on double that price last summer (Paul's Cycles,
Evans,
Wiggle etc are all worth scoping out for such deals).
What will you be using the machine for, your age, ability, intended mileage, any injuries etc?