Thanks Gnasher, before you posted I was looking at the price difference and it's only 26 quid exta for the kit with with pistons. So I can be as rough as I need to be in getting the old ones out.
Just to confirm that I pop them out without splitting the caliper
If you've not got piston pliers, be sure to leave enough of the second piston proud of the caliper body that you can grip it. These pistons are quite long and you need to push which ever one you're not taking out back into the body, just make sure you do as above or you'll have to get hold of piston pliers to remove it.
Push them both out with a single pad between them, get them a equal as possible, now clean the pistons ensure you get to the undersides, grease them and push them back. This will help free them, it will also give you an indication if they're seized, from there you can decide to split or not.
As mentioned these are often a real pain to remove and they can lock themselves in, I've had many an owner turn up with a caliper body with chewed up seized pistons.
If your's have been well maintained your should be ok, good luck.