The penny has dropped!
What was puzzling me was that the "pump" piston is the same displacement as the combustion piston. So it wouldn't seem to do much supercharging. And how did it pressurise the receiver?
The answer is obvious (eventually!)
The combustion cylinder is a four stroke, needing filling only once every 2 revs. The pump can pump its displacement (sorry madam) every rev. Hey presto, pressure builds up.
Next question. How does the phasing affect the pulses?