Update:
Had the rear end inspected, all bushes ok, Anti roll bar and bushes ok, No squeels from wheel bearings. Rear wheels where out of balance, and now re balanced, all seems ok. Borrowed a rolling road for an hour and car driven at 50Mph where the shudder happens. Went into pit with techs and noticed, while the undertray was off that the near side inner drive shaft wobbling a bit. Once stationary, there is significant play in the coupling, off side seems fine. Have ordered new component parts from Mangoletsi, who promise parts to there customers in 3 working days.
Once fitted will see what difference this has made. Mangoletsi have advised me that there are issues with drive shafts and the spider coupling, it according to there excellent service team, says it happens around 45,000 miles to 65,000 miles, mine has 66k on the clock. They have said that the Vectra C and Signum cars have had simuler problems at this mileage with shudder and vibration, are are repaired with new drive shafts, not with recon units.
Considering this has happened to me and Alan C, it may be wise to other owners of this forum, when you next under the car or having it serviced, get your inner and outer drive shafts inspected as precaution prior to getting the shudder effect.