Hi Graham

several scanners have a high pressure fault finding routine listed for this engine in actuators or special/service functions. i've never used it personally, but not for any reason.
i think some of these vivaro have an electric low pressure pump in-tank & a primer bulb, if you can see it & there's only 2 wires in the multiplug for the sender wiring & tank then it hasn't, if it's got 4 on both it has.

if the rail pressure relief valve has a leakoff pipe attached to it crank the engine with the injectors electrically disconnected, if you have fuel coming out of that pipe from the rail it's sticking & losing pressure.

have you done a cranking leakoff test on the injectors both hot & cold?

as for the shaft breaking in the hp pump, i've only personally seen that in the earlier 1.9 engine.

Steve