Just to add a little more information to the mix.
Andy (Grimmy), who was riding the bike in question, works on a gas rig in Liverpool bay, two weeks on, two weeks off. He bought said 450 not long ago, and has had all sorts of electrical issues with it as the P/O had "fitted" an aftermarket speed/tach/temperature gauge and butchered the wiring. Every time we got out for a ride, the thing would misbehave, but all the cycle parts seemed in good order. Last week Andy got back on Tuesday, took the bike in to an electrician, picked it up Friday, put fuel in it and loaded it ready for an early start on Saturday.
The cable broke in the middle of the two mile stretch of bog otherwise known as "The Motorway", far, far from anywhere. I did wonder about the possibility of dumping the bike there and coming back the next day. No one would know it was there, only us six!
Loz, Timpo and Andy spent nearly an hour coming up with various bodges and fixes, while Forry filmed, and Steve and I smoked.
The decompressor lever bodge worked well, but think about trying to ride another mile of bog, with your index finger as the throttle, your remaining fingers for the clutch, and a throttle grip that spins freely.

Best quotes of the afternoon, as we started off again:
Andy - "this is going to be dodgy!"
Steve - "not as fcukin' dodgy as pushing the fcuker"
and after we made it to the end:
Forry - "have you seen how fast Andy got through that?"
Steve (again) - "you think he had a fcukin' choice?"
Next day, Mike rolls up with new cables, and it's only while fitting those does it become apparent that at some point in its life the cable mount on the carb must have taken a knock. With the new cables installed and adjusted up, the inner is sawing on the side of the quadrant and the action is very stiff. The locknuts had to be left loose to allow an easier lead onto the quadrant, and to allow the throttle to return freely. (we met Mike at the start of the next days ride, or it would have been properly "adjusted" in my garage.)
Needless to say, my throttle cables got checked over that night!

Mark