Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Disconnect wire marked "SL2 IN" from the mains terminal block at top left of PCB housing and the wires marked "WEATHER COMPENSATION" at top right of PCB housing and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the boiler now operate correctly?