My Simpson esprit 500 washing machine isn't spin drying properly?

My Simpson esprit 500 (5kg) washing machine isn't spin drying properly. It spins, I'm not sure if fast enough, and it seems to pump all the water out but the bottom clothes so I assume the heaviest clothes are still fairly sodden at the end of the spin dry cycle. The belt seems to be ok but is it possible it might need tension as it does seem to have a lot of free play and if so how do I adjust?