Stoic, an interesting point about definition of "congested". In short, there isn't one, or at least the CAA consistently refuses to define it and it is certainly defined nowhere else. Thus you can, in the UK, be prosecuted for breaking a definition that you cannot know as it does not exist! How's that for justice? In reality it seems that an area is congested if the CAA prosecuter says so, he may take advice from a borough surveyor for instance, (notable experts on aviation related legal matters, of course) so it is effectively a personal, unique and somewhat arbitary judgement. Totally against every principle of the law, but that is how it is!