Set Appliance State:

CH Demand
Isolate Mains Power to appliance as per TOPs P4
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?