Lee
the belt is called an ancilliary drive belt or 'serpentine' belt
Fifteen quid or so from Eurocarparts.
Loosen off the one that looks like it's not doing anything - that is the tensioner - push it back a bit away from the belt - take off the belt and reverse the process.
A TVR dealer in Sussex once charged me 125 pounds for doing this - glad he went bust.
make sure you get the correct part number - written on the belt.
no mystery to it - should take 15 minutes.