The reason it keeps coming of the rings is that it's not been set up properly. If you had it set up by the people at Halfords then I am not surprised. Take the bike to your LBS and get them to give it the once over and running properly.
Took it to the LBS this morning and they sorted the gears, the rear mech was bent from my fall yesterday Have 't ridden it yet but I do totally trust they've done it right.
The question that keeps popping into my head is 'Should I have got a Specialized?'. I know I could get a refund still and go elsewhere but the difference in bike I'd be able to get is the Boardman fi Comp or a Specialized at about £650. Not sure if I should do it, maybe the bike would fit me better...
I don't know enough to compare the components on both bikes and know I've got an alright deal.