Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Summer/Winter Switch set to Summer.
Remove connector 6 - 12 and connector 17 - 23 from PCB.
Remove two red wires from DHW thermostat (marked P1 and 2)
and connect these together.

Measurement:

Measure resistance between the following connectors at the end of the disconnected wire NOT on the PCB:
HW tap OFF.
Connectors 8 & 9 should be Open circuit.
Connectors 8 & 10 should be Short circuit.
HW tap ON.
Connectors 8 & 9 should be Short circuit.
Connectors 8 & 10 should be Open circuit.
Reconnect red wires after test.