Cheers guys, will try the "pulling the chain off at 9 oclock" thing. But will just get a couple of new chain rings anyway as I think its probably well overdue!
Another thing I cant understand, When I got the bike initially, I was able to use all three chainrings, and get full range on all the cassette cogs, but the bike got stolen and returned to me, and needed a new chain and front mech. I got the bike back and since then have never been able to get onto the big chainring. Ive changed the chain once before to try and sort the length out, but if its loose enough to get onto the big ring, at the lowest gear, ring combo, its hanging loose. Cant understand it. Cant even remember if they changed the rear mech after it was stolen, I suppose if they did and fitted a short cage as opposed to a long cage, that would cause the problem?