Hi Phill, P0089 Fuel Pressure Regulator 1 Performance, if this was a Denso system I would say that the new IMV you fitted needs adapting, “Fuel Pump Learning Value clearing” without this the idle pressure will be height at 500 bar.
But a Bosch CP1H system I would check the fuel feed and return. Can you scope the mass air flow sensor and the rail pressure sensor at 2/ms per div from cranking to idle full throttle back to idle to off, and post it?
Regards, Martin.