Thanks Christian

I realised after I'd posted that I left out some detail but you have
added it.


This is slightly different from my previous "ideal" (number in bracket is
the difference), but should still work perfectly. The effect is not to start
the hot water pump directly in response to a hot water heating demand.
Instead, the system waits until the boiler's pump output is activated, which
should happen as the BIL input to the boiler has activated.

However the conventional single pump system does that anyway!

I bet someone comes up with a system that doesn't use a relay at all, but
just changeover thermostat terminals...

I think you need a doupble pole switch somewhere to make it work. Can
be either thermostat though.