A 3-way sounds good.. I’ll have a look around…
So I should wire it up to use the White wire (for now at least) and run whichever valve through that then?
I would have thought a 40w iron would be hot enough? I’ll have to keep trying I guess… I only just bought this one, can’t afford another 
If all else fails, could I drill a small hole in the chassis and use a screwed solder tab like that for the inputs’ shields?