Well, I got a reply from Benny which was somewhat helpful, however I now have some more information from someone who has recently done a Petrol Engine Management Course at the Vauxhall Academy in Luton.
He tells me that there is a known problem with the Z22YH fuel pressure regulator, the symptoms are similar to what I've seen and it seems that the fault is due to the regulator overheating (happened first when I was in a long slow moving traffic jam) and 'sticking' in a state which allows the fuel pressure to decay to the point where the fuel rail mounted pressure sensor signals the ECU which then appears to shut the engine down.
A temporary fix until a proper fix is available is to disconnect the pressure sensor, apparently the engine will run even if the high pressure pump has failed as the lift pump output pressure is enough at about 3.5-4 bar. Of course, there is some risk I suppose, maybe poor fuel consumption or perhaps worse, but I'm going to assume that the pump is OK for now.
My local dealer in Bedford had 4 cars with the same engine in yesterday all showing the same fault. They didn't know about this problem's cause until I told them, even though they had spent several hours in total on the phone to Vx technical.
Interesting I thought, I hope Vx can find a fix for this soon, I reckon that attaching the fuel system to the engine block is one way to overheat it!
I note that the regulator has not been changed on any of the 3 occasions when the fuel system has been worked on on my Signum, so maybe some of the replacement pumps were not needed and the fault was resolved when the regulator worked apparently normally when the new pump had been fitted.
Anyone else have any further information on this?