The taper is fine. Somehow the nut got loose from the torque on the crank. I spotted it, removed the (horrible plastic) cover to check and decided that rather than walking I'd try and nurse it home gently. It was just out from the LBS because of a loose bearing in the bottom bracket. That problem is now sorted, but they must not have torqued it on tight enough. I'm tough on bikes - especially cheap ones (even if they are new) so I expect some problems from time to time. Fortunately no damage was done and its back in business. Still, the system will always find the weakest link and its not always where you expect it to be. Of course it had to be a day when I supposed to meet my kids early.