Jon, EPC is showing that the different models of Vectra B's were fitted with five different sizes of rear anti-roll bars for different type chassis e.g. SRi models had a lowered sports chassis (one ARB is no longer replaceable).
15mm - part # 90495587
16mm - part # 90495647
17mm - part # 90497791
17.5mm - part # 24455460
18mm - part # 9191934 (NLS use 24455460)
There were only four sizes of bushes fitted though - part #'s 90373771, 90496706, 90497793 and 9191933.
Now the interesting thing on EPC is that it seems to show two different lots of part numbers for the brackets (so they too are possibly different sizes depending on the size of anti-roll bar and bushes fitted).
These are as follows (part #'s 1 go together and part #'s 2 go together):
Bow, Bearing, Rear Stabiliser Shaft - part #1: 90496703 ; part #2: 90575574
Plate, Bearing, Rear Stabiliser Shaft - part #1: 90496704 ; part #2: 90575575
I would suggest ringing your local Vx parts department, with your VIN No. handy and ask which size ARB was fitted to your Vectra CDX and verify the bracket part numbers required.
The brackets should not break, so maybe you have rear suspension alignment issues. Have the rear trailing arms been removed? If so the vehicle will require rear toe alignment which can only be done using a special GM tool (Kent Moore KM-900) even if the trailing arm front bushes have been replaced with Powerflex. If you have 4 wheel alignment done all it does is check the front wheels are in line with the rears, so if the rears are out the car will 'crab' on the road.
Hope that helps.