Its ok as a budget and you'll get an ok bike, and Halfords are an ok supplier to use - I'm a big fan of not spending one penny more to achieve your goals but also a victim of then going back and spending much more than I intended to in the first place
I'd prefer to buy new rather than second hand simply because some bikes can get hammered especially if "younger people" have been using them on rough ground or riding up kerbs etc - I saw a great example of this when my mate got his bike out for one of our long distance rides a few years ago and found that his son had been using it, both wheels were buckled but better than that the gear cassette on the back wheel had torn loose from the hub and you could pedal the bike as hard as you liked but it wouldn't turn the back wheel one inch
Your budget is pretty low for buying new it has to be said but if you are using it for unmade paths, towpaths etc and not actually taking it down a mountainside then anything that Halfords have will do the job and not fall to pieces in a week, if it does then take it back, they are actually pretty good.
Tell them in Halfords what you are going to use it for and try and keep focused on their Hybrid bikes, you'll find a better seat position than a true mountain bike and won't knacker your back up quite so quickly, get as many gears as you can with as many teeth on the big front gear as possible and your on-road cycling will be much more comfortable - a big difference between the number of teeth on the largest of the front gears and the smallest of the front gears will give you lots of useable choice for both on and off road, flat and inclines - then learn how to use them properly and you won't throw the bike away after two rides wondering why you're so knackered.
Thats just a rough guide but I see so many cyclists commuting on decent quality bikes who are peddaling like windmills on flat roads and going nowhere fast just because they won't use the gears properly - think about how fast the peloton can ride in Le Tour with very slow lazy revs on the pedals - ok they are slipstreaming too but they have huge front gear wheels to help them achieve that - it makes a big difference.
For a bit more money have a look at what they currently have in their Carerra range, but most importantly, get them to set the bike up properly for you.