I have managed to fix it,!! Over time the belt works its way across and comes off, but to get it back on you have to loosen off all the bolts at the smaller wheel at the front, and then un hook the big spring below ,! Then you will be able to slide the smaller silver wheel up, so the belt can loop round that aswell, so it will come off the big wheel under the silver one then round the front one,!! But u need to pull the spring back and hook it on whitch isn't easy, because that's we're you get the tention in the belt, I used a big screwdriver through the hook and pulled it back to get the hook back in, good luck,!!!
Simo, March 2012