Citroen Xantia intermittent starting

Relay negative pull down is controlled via the ECU,
Terminals 1 and 29 if I recall right are grounded by the ecu to activate the relay.
BUT only when the ECU receives a cranking signal from the crank angle sensor.
and it wouldn't be the first intermittent CAS I've come across.
If not the CAS then you have a possible ECU ground fault.
or ECU fault.
Try bridging the fuel pump relay terminals so the pump runs then see if it will fire up.

Thank you,not quite sure I understand ! Under fault conditions I think I can hear the relays 'click' this is before the engine is cranked over

If the relays click then the ECU is grounding is working,
that doesn't mean volts/amps is reaching the pump.
So if you unhook the tank plug and connect a 25W indicator bulb to the ground/pos of the plug the bulb "should" light on crank,
That will test the relay,
the ECU and the wiring.
But again I'm still suspecting an intermittent Crank Angle Sensor fault.
These engines need a minimum of 180 RPM on crank,
some register has to show on the rev counter.
Is it?
Or better would be live data from a scan tool.
The rev signal comes direct from the CAS.
So get a scan tool on it and check the revolutions on crank.

double relay faulty

Many thanks; your information is all very helpful.
I'm still trying to find the fault. You say the engine has to crank to initiate volts to the pump,this I understand.But under fault conditions the failure occurs before the engine is cranked,ie when the'key' is turned to the last turn before the starter motor is called there are no volts across the pump( I have a volt meter across) under normal conditions volts appear for about 5 seconds then disappear until the engine is cranked,then the volts appear and stay there while the engine is running. In other words under fault conditions there in no point in cranking the engine as the pump has not yet been primed, if I do crank the engine it turns over well but does not start. I have yet to determine if the relays 'click' under fault conditions. Once again thanks for your help and for sticking with me.

yours

David Terry