Trev, I would use just about any high quality enamel paint after first doing a full cleanup with a scotchbrite pad and using an etching primer..
Standard etching primer will be fine Trev. The high temp stuff would only be required if going over 5-600 degrees F. You better be well below that...
Beyond 120 degrees C and you're going to have engine issues. I'd aim for everything being rated for 150 degrees which gives you plenty of leeway.
Hi Trev,
Another choice is on www.frost.co.uk Eastwood radiator paint, satin or gloss.
As it's suitable for traditional radiators, it's made for copper. I don't think it uses an undercoat either.
Cheers Steve
Get on with it.