First of all, you need to learn how to maintian a bike, just because it comes from halfords, doesn't mean it isn't any good, just it's been badly maintained.

For you 'info'

at your budget of £300 not suspension is going to be great, you will get basis front, basis good full suspension bikes will cost from £1000 up
frame material, at your budget you only have the options of Alu or steel, both will be low end tubing, doesn't really matter which at this price point.
Brakes, discs at this price point if they are mechanical are not a huge advantage over v-brakes.
18+ gears, at you price point, you should get at least 24 (3 x 8) or 27 (3 x 9) speed, 18+ is for sub £100 bikes

For the suggestions listed, would forget anything by Raleigh, it's a long time since they have offered good value for money bikes, this will change over the next few years as they have been bought out, but not for now.

Would look at previous years models which have been discounted, Pauls Cycles is good for this http://www.paulscycles.co.uk/offers.php?cat=1
Also Halfords have some models in the Carrera line which are very good value for money if you can strech to £330 (discounted) there is the Vengeance .http://www.halfords.com/webapp/wcs/s...0151&langId=-1

For your described riding style and budget, do you even need a mountain bike, ones which can actually go off road and survive will start at full retail from around £500, you description sounds like a hybrid would work well for your needs.