Hoover washing machine does not do full spin cycle - any suggestions?

My Hoover washing machine (special edition 4 years old - but had lots of use!) goes through the usual cycles, but will only spin slowly to remove water and when it should go into a fast spin the drum does not move round. The machine is making the right sort of noise, like it is removing water, but it just isn't spinning round.

In the spin cycle it usually starts off slowly then goes into a fast spin, now it starts off slowly and then stays still.

The rest of the cycle is working properly, except for this fast spin. Am I right to think it can't be the belt because otherwise it wouldn't spin at all?

If anyone has any ideas what the problem might be then this would be much appreciated. I don't know whether I should be trying to look at it myself (I'm always up for trying this one although I have never taken a washing machine apart before!), calling out an engineer (could be costly?) or cutting my losses and buying a new machine.

Many thanks for any responses.