Gave up on taking out the regulating valve. Just couldn't get to the third (lower) bolt without taking the manifold off. Tried all sorts of gymnastics with double quarter drive universals etc but no way I could get the torx engaged. So I went under the car and put a manual pressure guage on the downstream end of filter (6-7 bar with pump run on 120l/hr using Foxtel) Then pulled filter. Upended the filter into a glass jar. perfectly clean diesel, no water, no sediment and no metal flakes. I'm hoping that means my HPFP is ok.
Not exactly sure what to do next. I hate to swap parts without a solid diagnosis.

Rail pressure regulating valve off back of rail?
Control flow regulating valve off pump.

Seems like I have to remove manifold for either which also means disturbing a million other connections.

I'm concerned that i'm missing something else which might be leading to the DME downregulating the fuel pressure.