Halfords have gloss black enamel paint, £5 for a 250ml tin. Just used it to touch up some engine parts on my Honda Blackbird that I'm getting back on the road, it was just what i needed and came up on Google search for gloss enamel paint to touch up the wheels and some bits on the engine casing. It'll withstand the heat and petrol won't wash it off. While i wouldn't normally recommend Halfrauds for anything but fluffy dice for a boy racer mobile, you are uk based, and stores are open. Nice amount in the tin will see you good for touch up for years and it's a fiver.