try pulling out the ignition barrel (quite easy actually) and checking all the connections are still soldered on fine. Had very similar problems, especially not switching off but only with kill switch ! I bet you any money that the ignition wire has come loose. If you can't solder it back together (can be a bit awkward !) you can get a whole new barrel and fit it easily enough from M & P, www.mandp.co.uk.
Jake, August 2007