Hi Shaun,
As you know diagnostics can be about eliminating the possibles until you get to an answer.

I would carry out the tests that I mentioned just to take them out of the equation.

I would test the supply line for a depression of no more than 100mb at the pump (vehicle running).

The vehicle should be fitted with a VP37 and the cavity pressure is checked on the outlet port of the pump (before the outlet restrictor)using a banjo adapter and a pressure gauge.
The pressure should be about 5 bar at idle raising to 10 bar as the rpm increases.

The PWM signal can be measured at the connector plug on the pump.(remember that the frequency as well as the duty cycle of the signal may change as the rpm increases but it should be stable at a set engine speed and load)

I hope that I am not "teaching you to suck eggs"

Regards Mark