It can actually all be done with a single relay (with Double pole change
over contacts). I have been working this out over the past few days.

Relay coil goes across the CH Pump.


Boiler overrun contacts.
Boiler overrun live goes to NC contact.
Heating Demand Live geos to NO contact.
HW Pump Live goes to Common.

Boiler control Contacts.
Heating Demand Live geos to NC contact.
Unswitched Live goes to the NO Contact.
Boiler switched Live goes to common.

Someone may be able to do an ASCII art version of this, It's beyond me.

Christian McArdle wrote:
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. However, it is essential
that this doesn't end up as a call for heat back at the interlock, or it
will never switch off. You also don't want the HWC pump to run when the CH
pump is running, or the HWC will overheat.