So from here, http://www.vectra-c.com/forum/showthread.php?t=63156, the balancing is now OK, the alignment has been checked and found to be spot on but the steering wheel is still off centre to the right when the car's going straight ahead.
Now some might say that I should swop the front n/s and o/s wheels around but the new Uniroyals have rotation arrows on them. So that put paid to that idea.
I might just swap the fronts for the rears and see what happens.
But if the alignment is OK, what causes the steering wheel to be off centre?