I've been riding a Carrera Vulcan for the past few years, it's a descent bike. I'm starting to do rougher rides though and think getting a full suspension bike will make the ride a bit more comfortable.
Looking at spending £500-£750 second hand.
Just come across a 2011 Specialized Pitch Comp for £550 I'm tempted with!
Get yourself on eBay, loads of bargains there. You obviously need to look at anything before parting with your money, but a lot of people buy bikes and never use them. Something mainstream that's been looked after and you should save a fortune.