On another point, I seem to have the chain rubbing slightly on the front mech if I am on the large or small gear at the front and the opposite on the rear cassette (does that make sense?) I have tried adjusting the front mech but there just doesn't seem to be enough room between the cage on the front mech, is this normal?
Don't ride with these gear choices. If you are at one extreme or the other at the back, you will almost always find that there is a gear ratio using the middle ring at the front that will give you the same output for your effort. Riding with the chain at these extremes will damage the chain (premature wear) and you run a greater risk of dumping the chain off the chainrings with it running like that. Ultimately a roller chain is at it's most efficient when it is running in a straight line. Try to keep it that way, and you shouldn't go far wrong. Certainly with a triple at the front and an eight speed at the back, I would try to stay well away from using the inner 3 rear gears with the front big ring, and vice versa. If you feel that you need to use those ratios, select the middle ring at the front, and look for a rear gear toward the middle of the cassette. It's a little like driving your car in the correct gear, you just need to learn your way around the gears on the bike, and develop your inner sense of mechanical sympathy. It'll soon come.
Quite a few modern MTBs with ten speed rear cassettes are now running doubles at the front, because it reduces the amount that the chain needs to 'twist' between the extremes of gears, and although you think that there are fewer gears (20, as opposed to 24,27, or even 30), there are just as many 'usable' ratios, it's just that on an 8 speed triple (24), some of the ratios are duplicated on an adjacent chainring.
Hope that helps.
Whatever you do, don't bin it!!
^^^What he said^^^
Quality piece of iron, well worth some TLC.