I think some recommended Diamondback Mojito as a good and cheap bike, 29er, SS and somewhere around £300. Aluminium frame, ss, cable disc brakes, so You get the benefit of disc brakes without the faff of bleeding etc. However for £300 You will get a lot on eBay but as with second hand bikes there might be some work needed or something might be broken etc and the bike will end up more in the end. As Al said, look for some cheap on-one bikes as well.
My plan for the winter is a a £50 bike (ended up costing about £100 with all the bits to get it working) as a commuter, road bike with mudguards on for road rides and training, MTB will be just for occasional use on the odd ride with mates.