Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Disconnect 240V external control wiring and make safe
Link out 240V external controls with a wire link
(RT terminal to L terminal)
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With the 240V external controls bypassed does
the boiler respond to a CH demand?
Note.
If boiler still does not respond leave external controls linked out.