Scooterotica

you cannot use your multimeter for an insulation test you will need a mega ohm meter, even then it wint tell you where the fault is, just if there is an insulation issue, but bt using process of elimination you can find out by disconecting everything and testin 1 bit at a time till you find the culprit.
me personally would fuk the fuse off out of it and see whats what in a blacked out room, as you might see some sparks, or smoke in daylight, but you run the chance of wrecking other stuff

If it is a short blowing the fuse then the fault will be able to be seen on a standard multimeter. I wouldn't advise to ever bypass the fuse as you could melt the loom or cause a fire.
I would remove the fuse, to disconnect the machine from the battery, and connect the multimeter on a high ohms setting between the red wire to the ignition switch and earth. Turning on the ignition, but with all loads switched off, the meter will test all power wires up to the switches, there should be no reading on the multimeter (you will have to disconnect the neutral indicator wire from its switch and insulate it first or you will read through the indicator lamp). If no reading is immediately apparent then try moving the handlebars and the gearshift keeping an eye on the meter to see if the movement causes a wire to short. If the fault isn't found here then you will need to start testing each circuit beyond the switches, remove the loads (lamps etc) and switch the circuit on, again there should be no reading between the ignition switch feed wire and earth.
As your fault is so intermittent it may not show up at the time of testing so do a visual check of the wires wherever they may suffer damage from movement such as where they enter the headset or where the wires go into the gear change (this is a known weak point).

Just on the off chance, have you checked that the loom isn't pinched under the tank or anywhere else? Have you looked all along it for any signs of damage? It's just that you mentioned that the fault showed up when decellerating made me wonder if a weight change was causing pressure on the loom anywhere. You are clutching at straws after all Maybe have a look around under the bridge piece. Good luck buddy

OK thanks for all, all advice is really welcome so cheers.

I'll let you know how I get on and fingers crossed.....don't you just luv'm!!