First things first: on the left is the old style caliper, on the right is the late spec ATE caliper as fitted to Astra GTE 16v and MK3 Astra.

I bought remanufactured calipers for £50 each exchange. The other option is to fit new seals to old calipers, however I've never found this to work too well mainly because the bores rust up. Remanufactured is the way towards an easy life. The picture on the left shows a caliper with two pads in position. The missing part is the carrier section....

This shows the carrier section. It bolts directly onto the mounting holes in the suspension knuckle through the two obvious holes. The slider pins are attached to the carrier and the caliper body runs on them. This is one of the main reasons why the ATE calipers are better than the earlier sort. The sliders actually slide! The other reason seems to be due to the retaining spring, shown later.

This image shows how the sliders fit into the caliper. They just push into rubber bushes. There was special grease supplied with the calipers but Copper anti seize grease will work as well. There should be a rubber bung on the end of each of the rubber runners.

I also had to pick up some fixing bolts from my local Vauxhall dealer and some Copper washers for the brake line connections. There's nothing particularly special about the bolts. I have reused old washers before and it's 50-50 as to whether they seep or not. Not a good idea on safety critical items like brakes.

I was told that getting hold of the carrier sections would be a problem. First off I contacted Maynards where I bought the calipers. They said they'd have a go but weren't sure that they could get them. So in the meantime I bought two used calipers from my local salvage place for £25 each. Unfortunately, Maynards phoned me up the next day to say that the carriers were coming, £20 for despatch.
Getting the sliders off for cleaning was a bit tricky, I had to resort to the blowtorch and a long extension on an Allen key (as shown).

I soaked the sliders in Wonder Wheels alloy wheel cleaner for an hour before cleaning them with some Carb cleaner. Seemed to work really well that.

After that the sliders were refitted, greased up and the carrier and caliper sections combined. Then the pads were fitted (which came free with the calipers from the scrappies). I smeared loads of Copper anti seize grease around as well. Obviously it's not a good idea to get grease on the pads.
Pay attention to how the retaining spring fits - I put it in upside down first time. I noticed later on. That spring is another part of what makes these calipers good - it seems to push the caliper open when the brake pressure is removed.

Finally, the old calipers were ripped out and the new ones put in their place. A top tip here is to make sure that the bleed nipples point upwards - I got this wrong once on my previous car. If you have normal rubber hoses then use a woodworking clamp to squeeze the pipe. That way you won't lose much fluid when transferring the pipes over. In my case I had to use speed - I hung the old calipers up, fitted the new ones, and then whipped the pipes over. I didn't lose too much fluid.

Finally the brake system has to be bled out to remove any trapped air. Providing that not much fluid has leaked out then a front only bleed might work. Get a one man kit for a few quid. Or so I thought, my first trial of the brakes took the pedal to the floor. So I bled the whole system out properly.

New discs with new pads will take a while to bed in - it took a few days to achieve really good breaking. In fact it was shocking for the first 10 times, leaving traces of the protective lacquer on the discs doesn't help.