Instead of fitting a fuse, I fitted a 5 amp Auto Reset Circuit Breaker across the terminals of the isolator switch. If the ignition switch is turned to the start position whilst the isolator is open, you hear a continous clicking noise whilst the breaker trips and resets.
The beauty of this arrangement is that you don't have to replace a blown fuse and the radio, alarm etc remain powered up even though the isolator is open.
These breakers are about £1.30 each from Maplins Electronics.
Gerald