Agree with the other suggestion.....if a spark results when you turn the key, the contacts in the switch might not be clean and making bad contact. I have had my switch apart at least ten times, they get moisture in them , it corrodes the contacts, or the soldered connections in there get broken. If you rule out all other posiblities, then mark and remove all wires from the back, check the wires themselves, get out the book and see which wires go to which accessory, and then use a volt/Ohm meter to check that the switch makes a good connection from say , the ground, through the switch, towards the park light, head light, and ...yes...the ignition. Unless you are like me, and will take "Anything" apart, best to get another one if you find some connection bad. Mine has been soldered so many times inside...there a springs and a ball bearing in there too. You CAN take it apart, you have to bend the crimps in the housing out of the way back there, so you can remove the Bakeolite disc with the bayonet connections from where it sits in the back of the housing, but it is a job for either a careful and methodical person, or...maybe a watchmaker. I have done it many times, but then again....I take the electronic stuff apart on the car too, and not everyone is cut out for that sort of recklessness.
Just reread all the entries.....hope you get the bolt out, it's kind of small for a drill and remove action, I had it happen in 74 and since then I have been rather careful about those funny looking screws. All in all, it sounds like you have a broken connection somewhere, bet you find a broken wire, or dirty connection somewhere. Every year I have something like that, last year it was a rear brake light switch that somehow sent all the power to ground when I used the brake,....must admit though, the battery was 8 years old and didn't give enough power to the Boyer, so the Boyer died from not having enough current every time I used the brake. Boyers need a good battery....if there isn't enough power to make the headlight bright when you turn it on( with the motor off), then the Boyer won't start the bike.....also something to look at. It was though.....very exciting to go into a corner and lose the motor a few meters before the curve. Took the switch apart and fixed it...problem gone. Be patient with your bike though....despite all this trouble, it will turn out to be more fun to ride than any of the rice burners......the Norton just isn't for everyone though, you have to enjoy tinkering...