OK. First, I think this maay have been posted in the wrong place. Oops

But, got it sorted. First, penetrating oil. Liberally.
Wait one hour.
Unclip sensor from loom so that the entire header pipe plus sensor can be detached.
Apply 1 by 22 spanner, header pipe held firmly under one knee, a bit of a grunt, and voila, it comes off. Yaay ...

Now to get the darn things clean. Lots of elbow grease, spray oil, some common old household brass polish ... well. they're a bit cleaner, but still black at the top of the pipes.
Tomorrow will try Autosol (if I can find it), after trying the harpic concentrate trick (again, if I can find some).

Woohoo, coming along nicely.