Stopped in at the Yamaha dealer on the way home yesterday, and asked the helpful guy at the parts counter what they used. Would you believe a freakin' broomstick!! He said just jam it in there and hold it tight while you undo the bolt.
Got home last night, grabbed my handy broken broomstick (which now has 1001 uses) pushed the fork upside down onto it and leant on it a bit and the bolt came out with absolutely zero effort!!
So there you have it, a magic broomstick.
Thanks for your help though DJRacing, it's been much appreciated.