It is a spring loaded pully.
Take the radiator fans of the radiator to give extra room
If the spring has gone then you will need a new pully The whole pully bolts on to the engine block
The pully itself has a bolt on the front of it for releasing the tension. Us a spanner to release the tension remove the belt, replace belt, release tension. It's dead easy.
You can remove the radiator for additional space
Undo jubilee clips holding pipes on, remove pipes, 2 10mm bolts at to of rad hold rad to front cross member. once these are removed the radiator falls backwards towards the engine and is then lifted out from the two rubber bungs that it sits on.
If you do remove the rad, remember to bleed it when refilling (Bleed screw on one of the two pipes going to the heater matrix at the bulkhead.
All the best david