My first post on this site so here goes.
After getting a PM from Boxer on AOC I decided to answer his question here instead of via PM as it may help others who are looking for the same solution.
First off the black plates Boxer refers to as backing plates are actually the carrier hanging brackets, these are situated behind the rear bearing and are locked into place by the rear bearing studs bolting through the rear beam, you can see the large hole in them where the bearing slides through and the 4 holes which the bearing studs locate onto the rear beam. The 2 holes near the edge are where the caliper carrier bolts too. I'd like Boxer to measure the distance between the bottom and top edges on those plates just to compare them to an Astra bracket, they should be identical but best to make sure.
The calipers are Mk5 ones, there is an extra piece of metal on the handbrake cable clamp which the Mk4 caliper doesn't have. The carrier is also different in that the Mk5 bolt holes are slightly thinner than the MK4, this means that the MK5 calipers won't fit the MK4 without you either using MK4 carriers or spacing the MK5 carriers out the correct distance needed. I had some phosper bronze spacers made which are perfect, just slip them onto the inside edge of the carrier and it fits. I glue mine in place so that if the caliper is removed they don't just fall on the floor and of course roll down a grid :crazy: The extra piece on the cable arm can just be cut off, the cable clamp where the cable nipple sits also needs to be squeezed a little as the MK5 uses a bigger nipple on it's cable so a quick squeeze with a pair of mole grips solves that problem. The caliper piston has been redesigned but it makes no difference however the MK5 doesn't appear to need the damper mod which MK4 calipers seem to require, whether this is due to the new design I can't say.
The 2 bolts in the pad picture are to be used in place of the 2 existing bolts which you remove in order to split the caliper from the carrier, they are precoated in loctite cement so it's a good idea to use them.
Hope that answers the question, if you want 4 of the phosper bronze spacers let me know, they are £2 each or £10 for 4 inc P&P.