Indesit WI101 not spinning but motor turning. How to fix?

My Indesit WI101 no longer spins but you can hear the motor whirring when it is on a spin cycle. I have tried several times to move the programme dial to the different spin options but all do the same: Motor whirrs , no drum movement, and then the dial clicks quickly all the way back to Start.

Is it a belt broken you think? Where do I start?