Set Appliance State:
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove black wires from thermistor situated on the right hand side of the boiler
Remove black wires from thermistor situated on the right hand side of the boiler
Measurement:
Measure resistance across thermistor.
Resistance should lie approximately between 1.6 kohms and 13 kohms.
Replace black wires.
Resistance should lie approximately between 1.6 kohms and 13 kohms.
Replace black wires.
Isolate as per TOPs P4
and refit as necessary