I had decided to do a full service on the calipers, which are probably hiding a lot of gunk and corrosion after 9 years.....and then I found 2 brand new Brembo M4.32 calipers on ebay for an R1200GS for £244 the pair. New pistons, seals, pins and clips would cost well over £100, not to mention the time. Add some new EBC pads for £60-ish and I have brand new brakes for about £300. I'll see how this works with the existing discs, which have plenty of life left.
Thanks for all the constructive comments. Hopefully this will take my brakes back to how I remember them from when the bike was new (or even better, with EBC pads) without spending a lot of money.