Hi Guys
Thanks for your responses, it is really appreciated. Find as follows:

Sean - I have run a fuel prssure test driving the car, and recording the results, and the pressures are spot on the specification under accelleration and normal driving, plus the fuel delivery is good, and easily exceeds the minimum limits, so we can eliminate this is a cause of the problem I'm afraid!!

Lee & Pete - Yes this is one of those with the MAP sensor on the bulkhead, and although the readings shown on the scanner appear consistent with the vac readings on my gauge, I now think its time to change it for a replacement, so this will be next on the agenda. Additionally, when we replaced the engine CTS, it was a non genuine part, and knowing how critical these are on the CTS, although it goes against the grain, I am minded to order one from the main dealer.

So guys, the saga continues as does the head scratching!!!

Thanks for your on going interest . . . . . any more thoughts anyone??

Roger