Kinda highlights the problem I guess. 

Should Iris have well-known constants defined explicitly in one place rather than sprinkled around the code base? I would've thought that radius of the earth was pretty fundamental to what we do ... I'm guessing that Cartopy might have it defined somewhere too?

Just worth consideration in the grand scheme of things. I'm not suggesting that we do something in this PR, but it just feels like a little bit of house-keeping might be in order at some point.