Indesit WIB101 - Belt problem?

Thanks in advance for any advice on this.

Drum stopped spinning a while back, first thought was that belt had snapped. Took back off and found belt intact. Managed (eventually as very tight) to get belt back on drum wheel and motor. The alignment seems to be off as now even on a manual turn the belt slips off the back (towards the drum).

Any advice would be greatly appreciated.