In message , Christian
McArdle writes
Does anyone have a diagram of how a system with 2 pumps and no valves is
wired together to give interlock?

Very simple. Wire the programmer output through the appropriate room or
cylinder thermostat to the pump. The same connections should also be made to
a relay. The Normally Open outputs of the relay should be connected together
in parallel and be used to run the boiler interlock.

Secondly how does pump overrun work if 2 pumps are used (i assume you
have to dump into the CH cylinder)

This gets a little more complicated. You basically need the pump overrun
output of the boiler to run the hot water pump.

So where does the heat go if the taps are turned off?

Didn't you mean the central heating pump ?

--
geoff