I'm thinking of using a Heatmiser PRT/HW programmable stat
http://www.heatmiser.co.uk/domestic.htm either in addition to (or as a
full replacement of) my existing programmable controller (it's LCD
display having died)

All their wiring diagrams show the use of a motorised valve to control
h/w and c/h, but my system is gravity h/w and pumped c/h, I phoned them
and they were less than convincing that the PRT/HW could entirely
replace the existing controller, I wonder if I can do it using an
external SPST/CO relay to negate the c/h pump supply when h/w output is
switched on? I'll probably email their tech dept to see if that elicits
a better response, but anyone here already done it or think/know if it
can work with or without a relay? Can I still incorporate a h/w tank
stat too?

TIA ...