Hotpoint Aquarius WMF760 belt come off

Recently my Hotpoint Aquarius WMF760 was making an odd noise while spinning at lower speeds. I had also noticed it had become very noisy at full spin. Today it stopped spinning.

I took the back off and sure enough the belt had come off. The problem is when I thread the belt back on, it just comes off again, if I keep turning the drum. It's as if there is a misalignment, but then I could just not be putting it on properly. Even if I try to force it to line up it still comes off. The belt comes off to the back of the drum wheel, so it ends up between it and the drum.

The belt has grooves in it and so does the motor. The drum wheel (I don't know the correct term sorry) is smooth. You can see where the belt has previously been on both but it doesn't seem to want to go back into that location. There is also some rocking movement to be had in one direction, if I try to rock the big wheel in relation to the drum.

Could this be fixed with a new belt, or is it likely something more catastrophic has happened?