Thanks for the responses, very helpful.
As for knowing what to call them and who, let's just assume a hypothetical flight from Headcorn to Elstree. If I wanted to ask for a clearance through the London City CTA (as it's marked on the map), then who do I call up? Thames Radar? City Approach? London City? How do you know?
Another one is all the MATZ's. They have no name. It just says MATZ on the map or CMATZ. I can think of at least one MATZ that has three different fields in it, so how do you know what to call them?