Many thanks Q
That will do nicely!
I'm afraid that I was approaching it from the other viewpoint - why shouldn't the selection be to ON for the OVHT. (I could, in fact, see why it wasn't NECESSARY, because of the output pressure monitoring) Once I'd started looking at it from that standpoint I was trying to argue the logic the wrong way round.
Obviously this living upside down that you chaps do down there in Oz is good for the brains. Perhaps I should come down and try re-charging the grey matter
B rgds