Hi Martin,
There it is then. Almost certainly I have a Mk1. On my machine the drive from the motor is a small pulley driving the wheel/pulley at the lower back by a 'V'-belt. With age this has polished, and probably stretched, giving rise to the occasional slip start. Possibly the very reason why Burgess changed to a toothed belt drive in later models.
Sorry I could not help, but thanks for the help with an identification problem.
I hope you get your small, but very useful, machine working again quickly.
xy