Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
CH Demand
Gain access to PCB (use Insulation Mat as required)
Remove the live, neutral and earth wires from the incoming side of the boiler terminal strip and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

With the wiring to the appliance isolated does the fuse still blow / RCD still trip?