[frankfront]

Further symptoms persisted. On long runs keeping the tank filled to the brim almost completely solved the symptoms. Nevertheless with the analyser plugged in the fuel rail pressure could be seen to drop on right hand turns but not left. There was a corresponding increase in the main pump duty cycle (tapped into white and white & black wires on pump head).
Conclusion being that the swirl pot was not filling properly or staying filled - therefore either the no return valve was stuck open or the there was a blockage in supply from the ejector pump/venturi system.
With the tank brim full fuel was able to slop over into the pump pot.
Opened the tank yesterday and removed the pump.
Where the Venturi return pipe tuns at the bottom of the pump body it had been bent and kinked during installation and stayed kinked. The result was to very much restrict the flow of fuel from the ejector/venturi loop.
The kind of plastic used and the thinness of the walls of the pipe lend themselves perfectly to this anomaly. Once the pipe is kinked it tends to collapse and stay kinked.
We fixed an external stent in the form of a piece of (proper) petrol hose and clamped it distally with a jubilee clip.
All symptoms have no disappeared.
Presumably in the process of fitting the first replacement pump something similar happened and the pipe was kinked in the same or a different place.
Examining the piping closely and the process of fitting I am surprised this doesn't happen more often. Decent Bosch pumps too. And careful mechanic - no problems in twenty years.
I will attempt to attach a couple of pictures of the kinked pipe.
Thanks to everyone who helped with advice.
Frank.