Hi Tom I don't know whether you have tried one of the larger Halfords, but they have quite a comprehensive selection, and if your "chip" matches one of their references they will mix while you wait for about a tenner.
Holts have a paint system to fill areasols to match. Our local accesorry shop has such a system [in essex]. A search of Holts web site should,ihope, find your local agent. Roy.