Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use Insulation Mat as required)
Disconnect external controls from connector CN5 & make safe
Link out connector CN5 with a wire link
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Cancel purge cycle & if required, reset boiler.
With external controls linked out, does the boiler now respond to a CH demand?