I don't know how much you want to spend or the type you're looking for but have you tried plasti-dip? If you just want it black then this will work and if you don't like it you can always peel it off.
Here's a vid on the concept.
Mittzy mate,
so by the looks of your overflow can it doesnt seem to need to be above the radiator to work ? Does the hose run to the bottom of the tank ?
@Wrecka overflow doesn't have to be above radiator. Hose just has to be on bottom of overflow tank. Radiator will draw back in via vacuum suction as needed.