Guilders paste is the kind of thing I'm thinking yep.. I get that real patinas are the product of the metal surface reacting with either the atmosphere or applied chemicals, however you can buy (in the US at least) chemicals/paints that mimic patinas and come in a variety of colours, and are called colored patinas (although they may in fact be more like dyes) was just looking for a UK supplier..
E.g. http://www.sculptnouveau.com/Results.cfm?category=6