Citroen Relay 2.2hdi cuts out - trouble code PO191?

My van when warmed up a little cuts out with EML on or goes into limp mode. When read, the code relates to fuel pressure which has been diagnosed as having too high fuel pressure causing ecu to shut the van down as a safety measure. I have changed high pressure main pump including fuel pressure regulator, in tank pump, fuel rail pressure sensor, fuel rail itself, fuel filter, engine ecu & engine wiring loop-all to no avail. The van can be driven until it warms up slightly then cuts out. my next step is to drain fuel tank incase any water in there......have run out of ideas now - totally baffled!!! would appreciate any suggestions / help