In reply to tlm:
Use Dulux Satinwood or another manufacturers oil based eggshell paint. You can get it mixed in any colour off the chart at a proper paint merchant, it brushes on easily, dries to a soft sheen and doesn't change colour or crack over time. I painted radiators with Dulux Satinwood and they still looked like I'd just painted them after 8 years. You can wipe it clean with a damp cloth and a little bit of detergent if you need to, without harming the finish. You'll need to rub the rads down with fine abrasive paper (180 grit or so) to key the surface and apply 2 coats, leaving 16 hours drying time between coats, but that or a proper radiator paint are the best way to go for a long lasting finish. I'd avoid any water based paints like matt emulsion, they're just not formulated to perform on a hot surface such as a radiator.