VICTA 505 2-stroke mower: self-propelled drive?

The self-propelled drive on my VICTA is not working properly. It will only drive if I over-tension the cable, and then tends to keep driving after you've released the drive-lever.

I've cleaned out the space where the belt runs and checked the gearbox and bearings are free.

Seems something is sticking: I can't see what it is that pulling the drive-lever is supposed to move. I imagined it was the idler-pulley just to the right of the big gearbox pulley but that doesn't seem to move when I pull the lever. Perhaps that's the problem!

Grateful for any advice.