Thanks tabetha, I took your advice and bought myself a haynes manual today to take a look at the wiring diagrams.

It looks like in most of the circuits this ING fuse is simply used to protect the Ignition switch (as you would expect) and is only used for turning on and off the ignition relay.

The typical scenario is

(Battery Positive) ==> (60A Fuse in Engine Bay) ==> (15A IGN Fuse) ==> (Ignition Switch) ==> (Ignition Relay) ==> (Earth on battery negative connector and Earth on Clutch Bell Housing)

Nearly all other circuits have a fuse after the ignition switch .e.g. the lights circuit is

(Battery Positive) ==> (60A Fuse in Engine Bay) ==> (15A IGN Fuse) ==> (Ignition Switch) ==> (10A Fuse for lights) ==> etc

This fuse after the ignition switch would blow before the IGN fuse if there was a problem with the lights for example.

Would it be correct to assume that my problem must be an exposed wire somewhere in first circuit above (i.e. the one involving the relay).

I took a look at the earth connector to the negative connection of the battery and it looked very dirthy.

Would a dodgy earth connection cause a fuse to blow?