Rider may be Roger Gledhill. Bike is a Yamaha DT-1 (250cc) or RT-1 (360cc),
and has a modified tailpipe, but no front mudguard, which is why he is covered in mud.
Part of the track must have been watered as this jump looks dry.

Thanks to Luderick Jacoby, Nomads Motorcycle Club of Cape Town, for this information