I had a feeling this would not help matters. I guess I need to see if the hose is leaking and whether the Aux air valve is working OK with the wiring going to it.

This will sound quite stupid now! I planned to reduce the RPM at hot idle but I could have sworn I read somewhere that I would have to readjust all sorts of things to keep everything in kilter, and was worried about altering the fuel mixture. At 11-10:1 it is quite safe however! It shall be something I sort the next time I pull it out of the garage (I'd work in the garage if there was space!).