Mick - I missed your "fuse blowing" comment. If the MAIN fuse blows as soon as you turn the key, you need to check the BLACK from your key switch and find out where its shorting to earth. (This is not a guarantee, but a likely cause). Common places are in the right side handlebar connections, and at the Regulator connections (left side panel) and lastly, the starter solenoid.
With a meter, and the key off, check for continuity to earth on BLACK. You should never find that to ring out. If this does not reveal the culprit, proceed checking BROWN and BROWN/WHITE. Because you stated the symptom occurs only when the key is activated, that tells me its within your switched 12v circuits which are the BLACK, BROWN and BRW/WHT from the key. And if the MAIN is blowing, then it should be BLACK as the culprit.