As planned, there is no hot water priority.

I suppose I could implement priority simply using a 3 way diverter valve and
then subdividing the heating port later on. I don't know if Honeywell have a
letter for this type of system. Although I've already got the three 2 port
valves, I'll have needed a couple more later on anyway, so the one freed up
won't be wasted.

Christian.