Thanks for your interest, but I've solved the problem which, commonly, was a very simple one. It was this: a new clutch lever doesn't come with a small, plastic bush. One has to remove the bush from the old lever and install it in the new one. But like a numbskull I wasn't aware of this because I'd thrown the old one away. It's the bush that depresses the cut-off button. But the mystery remains of why, when I depressed the button with a screw driver, the bike still wouldn't start. Oh well, at least I've leaned an important lesson: not to throw an old part away before having the new part successfully installed. I hope this may be of help to anyone who might one day find themselves in the same situation.