Geronimo W. Christ Esq wrote:

OK. This is good advice, I will do as you have said - better to do it
right and once, rather than have to go in later and start poking with it
again.

OK, the penny has dropped on how this works after reading the FAQ. The
output of the time switch is connected through the thermostat's control.
I figure then that the three-port valve would be of the three-position
type, such that when two signals are received at once it remains in the
mid position, switching to either side position when only one signal is
received.

I imagine some slightly less trivial is required for wiring the pump +
boiler, since they will switch on if either or both signals are received
but never if both are off ? I will leave that part for the plumber to
worry about.