There's a colour in the Humbrol enamel range which is close, although a touch light, but it's so long since I used it, I've forgotten the number! I used to darken it slightly, but normally I just use Grey No..27, with white added, until it looks 'right'. I always make it lighter than actual Neutral Grey, to allow for the scale effect. Sorry this isn't much help - I'm still feeling rough!