Am only able to get a general spec on this bike from pictures and forum chat, so here is what I think you have on the bike:
Full susser, with spring action rear shock. Shimano 21 speed twist grip gears. V brakes. Square tapered chain set.
The first conclusion of this bike is that the frame is heavy, while the components are not great. It has tried to put a full susser together on a tight budget. So there is a lot to improve. I am just wondering if the frame is worth it, but here goes.
As well as reliablitiy and longevity, well engineered parts are about weight. You pay a lot of money to lose weight. The lighter the bike, overall the better you like riding it.
The most obvious place to start upgrading is on the wheels. Rotational weight loss can have a dramatic effect. So, first step is to put some thinner tyres on there. Try the Panaracer Fire XC pro as a good all rounder. You might even be tempted by continetals for the summer ride. (£30)
The next option in the wheel department is to just replace them. The once you have will need something better. You could do a lot worse than a set of Mavic Crossmax wheels. Go for Hope hubs. These have nice bearings. £200
Get rid of the cheep chainset. You can get some 08 hollowtec II chainsets in either Deore or even LX for about £60 This could save you about 3-400g and is worth the weight. The bearings are smooth as well.
Decent bars will improve the ride no end. I would probably look at Easton EA50 bars with a slightly older EA70 stem. Combined you are looking at £60. However, you can get a Richley carnon bar and stem for about £70 from Merlin at the moment. (Carbon will absorbe much of the bumps that you normally feel from the trail and so makes the ride easier. It is also lighter, which helps). However, the personal fave at the moment would be an Easton EC70 for about £90.
Clip in peddles. These will help on hills and ensure that you stay connected on bumpy downhills. - Shimano PD520 £20. Shoes £30 for entry level.
Nicer saddle. WTB do a nice one for about £30.
Forks. The one you have is a bedspring. Go for an air fork with lockout. Say £300. This will seriously lighten the bike and make life a lot nicer for you. I would say you should consider a Fox Float or R100)
You might also look at a decent casset, however the wheel that you have at the moment is a 21 speed. These tend to be freewheels and so not casset compatable. However, if you are going to change you wheels, you may as well upgrade your gears as well (Go for altus or Deore). Allow about £150 for this upgrade.
As you can see. Upgrading can be expensive. It is worth it, in that you get better kit with better performance. The downside is that it is an expensive game. I have not gone for top end kit here. Rather, I have looked at a relatively mid range set up.
It is like anything though. You can make minor improvements that can have a dramatic effect upon your cycling enjoyment, like tyres or Chain sets. However, if it gets to the point where your shopping list of things to change gets expensive, it would actually be cheeper for you to just invest in a new bike. Up to you.