Set Appliance State:
On/Off Switch OFF
Set Multimeter to Resistance 
Isolate Mains Power to appliance as per TOPs P4
Remove front control facia from the boiler
Disconnect all leads connection plugs from the top of the control box noting their positions
Gain access to PCB by removing two screws that hold the control box to its support panel and lifting the box clear
Remove fuse F1 from its holder (and F2 if old style PCB)
Remove front control facia from the boiler
Disconnect all leads connection plugs from the top of the control box noting their positions
Gain access to PCB by removing two screws that hold the control box to its support panel and lifting the box clear
Remove fuse F1 from its holder (and F2 if old style PCB)
Measurement:
Measure resistance across fuse (both fuses if old style PCB fitted).
Isolate as per TOPs P4
and refit as necessary