So if it’s watertight, why doesn’t he call a vote of no confidence ?.....
Because parliament dissolves if an election is called, meaning parliamentary business could not happen. So, for example, if he decided to break the law and not send the request by 19 October, yes he could be put before the courts and possibly jailed for contempt, but parliament would not be sitting so they wouldn't be able to do what I said - take control of proceedings. And to be honest, he'd relish the opportunity to break the law and paint himself 'against the establishment' - as long as he gets no deal and he and his mates can start the race to the bottom, that's all that matters to him.