Both I and the RAC man tested the car and we both reckoned that it was the fuel pump in the tank.
Electrics to the pump seemed to test OK but no noise coming from the tank motor.
So new one needed. Not sure if they differ for model year though... i need one for a S2 2litre turbo, 1997 with trip computer etc.
I've looked at the citroen service site and it seems to be part "1525 42 Gauge with pump" although on the xm pdf file I have this is also labelled as "Bord Computer" - which I take as meaning it is the one that connects to the trip computer.
So i guess I'm looking for one off an S1 or S2 petrol turbo with trip computer. I am however willing to take another one and do a bit of DIY!

Suggestions and sources gratefully received.
Steve