In line fuel filter Fabia II 1.2tdi greenline

Help required with the in line diesel filter under the RNS of vehicle. I have the following issue,

 P008700 - Fuel Rail/System Pressure - Too Low Intermittent

The car starts okay, a couple of turns before it starts without any spluttering.

Acceleration is normal

Fuel economy is normal

No limp mode

Just the glow plug light which continues to flash until ignition off and on and it goes out.

The fault is activated during normal driving without harsh acceleration.

The engine bay fuel filter has been replaced.

My next steps are

One

In line fuel filter - OE number from VW part on car 1K0201051M. Can anyone tell me what its function is? Is it a filter or a regulator.

Can't find any reference to it needing to be changed as part of routine service

There are 2 inlets, one is blanked off and one outlet.

Taken it off and stuck my lips around the ports (yuk)

The port that is blanked off has no resistance when blowing through it.

You can't blow through the port that is connected to the tank.

Two

In tank fuel pump.

Should I be able to here the pump running with the seat up?

Any help appreciated