The problem is that when only CH is called the boiler will not switch off
when the programmer switches off. The same happens when both HW and CH are
called for by the programmer and the HW is satisfied before the CH.

The wiring seems to be fine and the mid position valve moves to the correct
position at all times and is correctly controlled by the room stat and the
cylinder stat.

Any ideas.

The only idea that I can come up with is that orange wire (which stays at
90v when there is no power to the valve after calling for CH only) is
somehow tricking the boiler into staying running and I suspect the boiler
PCB is faulty.

Adam