Set Appliance State:

Mains Power Off
CH Demand
Selector switch in CH & HW mode
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Reset appliance as necessary.
Allow boiler to run and burner to ignite.
As the boiler is lit can kettling / noise be heard from the boiler or is operation quiet before the fault reoccurs?
Note:
If the boiler does not refault click HERE to continue diagnosis.