I am upgrading my very antiquated central heating. I am fitting a two
channel programmer, two 2 port zone valves for HW and CH, and an extra
pump for the HW which is gravity fed at present.

As I understand it, this constitutes a Honeywell S plan, and, except
for the extra HW pump, is shown at:

www.octaveblue.co.uk/c_heating/sp_hw.htm

and:

www.octaveblue.co.uk/c_heating/sp_ch.htm

but these diagrams only show a pump for CH which seems to be wired to
run all the time whether there is a call for heat for HW or a call for
heat for CH.

Is there any reason why I cannot wire the CH and HW pumps respectively
between the room thermostat and the CH zone valve, and the cylinder
thermostat and the HW zone valve? Or can anyone give me a link to a
wiring diagram for a fully pumped S plan system?

TIA

Keith