Indesit WI 101 drive belt keeps popping off. What to do?

Help!

We have an Indesit WI 101 washing machine (about two years old) and recently it decided to stop working. On removing the back panel I noticed the drive belt had come off and for some reason refused to stay in place each time I put it back on.

Assuming that it was time to replace the belt anyway, I ordered a new one which arrived today. It fits on snugly, but when I turn the washing machine drum by hand to check it, the belt pops off again within less than one revolution!

The alignment between the motor spindle and the large wheel attached to the drum seems to be fine -- using a needle and thread as a plumb line I have checked this out and it seems the two are in almost perfect vertical alignment with each other, yet the drive belt refuses to stay in place.

I live in a remote area and the only people willing or able to fix it want £90 to do the job ('any job' - which in this case I assume is a fairly simple matter anyway), so I am desperately hoping that someone on this forum will be able to offer me some advice.

Thanks in advance for any help or information.