For some strange reason, the shift 3 no longer gives me a 'pound' sign but this ' # '. Does anyone know how to change it back? Not sure why it has changed or if I have pressed something accidentally.
Sounds like you changed your keyboard language to US instead of British.
I can't see why you'd want to go back to that wonky British, but you can do it via System Preferences-Languages & Text, Input Sources. Check the British checkbox. Probably you want all others unchecked.