Champion self-propelled rotary mower?

I have a 20" champion petrol rotary lawnmower (green), 2 years old, which I bought from Focus DIY. Exact model details worn away. The mower engine works beautifully, all cables are good and belts are tight etc, but the self-propelling drive does not work any more. There is no obvious adjustment and I have lost my manual, so cannot look the problem up. Anyone any ideas about the problem or even where I can view a manual. Thanks.