The belt tensioner has come off, and now I don't know how it goes back together. There appears to be only one way the tensioner fits into it's slots & I'm not even sure if this is right, but I can't find where to attach the spring.
This is a tricky one, but once you've done it, it will appear quite simple. You're right, the tensioner can only go on one way. The spring attaches from the tensioner to the swith located in the middle of the bottom panel. The switch has a groove in it for the spring. The drum belt has to be doubled up and looped behind the tensioner, and then onto the motor pulley.