I bought a pair of rear calipers form eBay 2 years ago fo less than £70
Link doesn't work anymore so unfortunately I can't share.
You can check realoem for associated part numbers and then search eBay for sellers. Some of these parts are used on other models so by searching for the part number widens the search items.
I get a lot of my car parts from Autodoc.co.uk (bear in mind £8.45 delivery if less than £140 order).
Just noticed you can now get a rear brake disc for £3.28 !!! (Maybe not good enough for the track days you do TheDan )
I have seen the ones from Autodoc and while this would normally be my preference, I don't fancy sending my calipers back to Germany to recover the £30 surcharge (per caliper). I will definitely be avoiding the £3.28 brake disc too!!!
I have searched the PN from realoem on ebay and there is so much choice ranging from £40 to £60 for a pair of rear calipers. Its the quality of these that bothers me most though as they are definitely a cheap chinese copy (although how wrong can you go with a cast blob of iron).
I have found a place on ebay that sells refurbed genuine calipers (and carriers) but they only have them listed as front + rear sets. Hopefully they can break them up.
I have already bought the discs (MTEC grooved - same as front) and pads (EBC blue for ease and cheapness).