Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Remove display pcb from front of boiler
Remove connector X40 from PCB

Measurement:

Measure the resistance between the wires on the connector X40 and the pins to the display pcb.
Wire 7 on the connector to the left hand pin.
Wire 9 on the connector to the middle pin.
Wire 8 on the connector to the right hand pin.
Expect continuity or zero resistance.
Note:
It can be difficult to reach the contacts inside the connector take care to ensure a good contact is made.