In reply to Tony the Blade:
For that budget I'd go secondhand, eBay is generally good and my most used bike shop.
Brand etc isn't really important. Unless you are short, get a 29er, it will be more versatile. Suspension forks make a huge difference off road but aren't essential unless you want to predominantly go 'mountain biking.' You can ride rough stuff without the, you'll just be slower and it will be a lot more tiring. This may not matter.
I'd look at Planet X/On One, Sonder and Canyon. You might find an On One Inbred 29er on eBay, a brilliant bike (I had one) if slightly crude and heavy but very cheap. Otherwise there should be no end of Specialzed Rockhoppers or their Trek/Kona/Giant equivalents at your price point.