halfords... about £7 for their proper heat resistant caliper paint...
make sure the calipers are VERY clean before you paint anything... all flakes, oil, grime, etc scrubbed.... again, Halfords, etc have grease and break dust scrubbing solutions...
most people advise taking the calipers off and buffing with a drill and wire brush attachment, but that REALLY does depend on the state of the calipers already and would be overkill on a modern car (I'm talking 10+ year old calipers and even a few Mk2 Golfs!)....
Where they've had decent condition calipers to start, I've seen some very good results with a good hand-scrub, a very carefully prepped caliper, well taped and delicately brushed... maybe not careful show'n'shine judge perfect, but durable and fine to the naked eye from a couple of feet away... dig out one of the how-to guides if you're struggling, there's loads on vairous forums.
main thing I'd say: whatever they say about drying times, even in this heat... leave it at LEAST that long... if you're gonna leave it to autumn, double it and winter, quadruple it and hope!