Give us a bit more info to work on Trev, do you mean the knob skips around or the whole machine dances about? Check the model too, they do have a APM8512 and there are 2 versions of that but neither have a start button.
need to plug in a module, probably that, but you will need the correct model number or the number on the module. Not a common fault on these but the timer is controlled by the module so i would guess its that.