Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Ensure that the boiler is not at its full operating temperature

Measurement:

Remove the Red (terminal P1) and Pink (terminal 1) spaded wires from the Boiler Thermostat (stage 1 function) and measure the resistance across the Thermostat (terminal P1 and terminal 1).