Diesel tank pump pressure?

Looking for technical help with the following please

Having an issue with the following fault on a 2012 1.2 TDI Greenline.

P008700 - Fuel Rail/System Pressure - Too Low Intermittent

The glow plug light flashes when it detects the fault when driving normally but no limp mode and other than the warning light no other indication of a problem.

I have replaced the engine bay fuel filter.

No change.

Next step is the inline fuel filter on the O/S underside of the car.

Before I change it I would like to check the fuel pressure from the tank to the filter and and between the in line filter and engine bay filter, before I change it and then again after I change it to find out if there is any difference.

Can anyone tell me what the diesel pressure should be from the in tank fuel pump.

Can anyone tell me the diesel pressure should be between the engine bay fuel filter and the HPFP.

Thank you