When I got up there, I found this:
i.e. looks like the result of some prior bodging.
I couldn't get a 10mm (too large) or a 9mm (too small) to get any purchase on this - bearing in mind it's quite high up. So I guess this will need a 3/8" - which I don't have to hand. (I didn't want to use any adjustables because it looked like that may been the cause of the problem.)
Given the state of the nut, I'd like to make sure I've got the right size. Can anyone confirm the proper nut size or otherwise describe how to turn this thing.
I'm guessing that these dishes are pretty standard. Here's a photo from further back:
Any advice appreciated.