Set Appliance State:

Mains Power Off
CH Demand
Isolate Mains Power to appliance as per TOPs P4
Disconnect one of the Red spaded wires from the Overheat Thermostat and ensure that the disconnected wire is safe and cannot touch any part of the Boiler.
On/Off Switch on.
Gas On
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Observation:

Warning!
MAINS VOLTAGE present at Overheat Stat and Disconnected lead during Test.
Does the Red LED illuminate and the Boiler go to a Lockout condition?
Replace Lead.