Hello Guyver1.
I have heard of people using Hammerite quite successfully.
However I have used Foliatec paints from Halfords, and that stuff stays on!! It's pricey though.
But whatever you use, the job is only going to be as good as the preparation. You'll need some masking tape, disposable gloves, dust mask, fitch type brushes of various sizes (be prepared to throw these away after), some damned good spirit-type degreaser, rotary wire brushes, emery cloth, a large bag of patience and plenty of time on your hands.
If you use the Foliatec paints it is critical to stir the paint pigment well. Two coats on each caliper should do it, and you will be pleased to know that one Folitec caliper paint set is just enough to do two cars.
Halfords also do their own caliper paint but I don't know anyone that has tried it.
I'm going to be doing mine before long, and will choose Foliatec again because I like the finish. It stays on, even if the preparation is not what it could be, and lasts for years.
Colour range
HERE