Thanks, Mark, Jon and Ian for the replies,
Much appreciated.
Ive done some further tests today.
Maybe the fault is developing.
I checked supply pressure to the pump, KOEO, and pressure did drop off from 1 bar to zero within 5 minutes.
But fuel remained in pipe but no pressure.
Dead headed fuel tank pumps at fuel supply at filter and pressure held ok.
Then tried clamping fuel pipe at tandem pump and pressure dropped off within 30 seconds.
Ran out of time today, but going to try and bypass fuel filter and check pressure drop. (Filter has feed and return , so may give odd readings ?)
Thanks
Joe