Someone nice has pointed me at http://www.volkspage.net/technik/ssp/ssp/SSP_334_d1.pdf
How it works
The electric fuel pump always conveys as much fuel as
the engine requires at a particular time.
The momentary fuel pressure is measured by the fuel
pressure sender for lower pressure and sent to the
engine control unit. If this pressure deviates from the
reference pressure, the engine control unit sends an
appropriate PWM signal (frequency 20 Hz) to the
fuel pump control unit. The control unit sends a PWM
control signal (frequency 20 kHz) to the electric fuel
pump until the fuel pressure returns to the map curves.