Treadmill in need of repair (West Totton SO40)

Treadmill is almost brand new as I used it four times before something broke (this is a picture of the replacement one because I forgot to photo the broken one). It's a mechanical problem not computer: the belt drifts to the right rapidly. The belt is therefore frayed on the right, but can be replaced or covered with duct tape. There are two tightening nuts, and I'm sure one of them has sheared off the screw. The replacement machine has given me no problems at all.