Ian has confirmed that the 2.5" spacing from mounting face to the outside of the rims remains constant for both front and rear.
So that would mean that being that I am going with 18" x 8" on the front, then offset for the front would be 1.5", and if using 18" x 10" rears, that the rear offset would be 2.5". I think I may even understand this.
We have gone for the 18" OM wheels with the charcoal grey (gunmetal) centres with polished rims. They looked quite nice at the showroom, just hope they go well with the impulse metallic (the current SS Commodore dark blue) colour we are painting the car.
Cheers