Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove black wires (1) from HW thermostat situated immediately to the right of PCB
Note:
If boiler has been running residue heat may cause thermostat to operate

Measurement:

Measure resistance across 57°C HW Thermostat.
Replace black wires.