Also, we wish to turn off the heating circuit off while the GSHP is in sleep without having to turn it off altogether - so that the time-counting for the pasteurisation cycles isn't affected. So by attaching a relay wire from an internal connector to an external switch, we can force the compressor off. I discovered that this does NOT actually turn off the auxiliary pump that sends liquid around the underfloor system (checking the temperatures) - the sleeping GSHP still sends power to the pump. In such a well insulated house, we just don't want it wasting 125W pounding water round and round all night, audibly, just to check the floor temperature while the GSHP slumbers.
So in this case, David advises that a 4.7 kilohm resistor can be switchable on and off on the bellwire that comes to-from the external air temperature sensor on the north wall. This will trick the GSHP into thinking that it is a hot summer's day, and will force the underfloor heating system to sleep.