"...it also sort of follows that Parliament is free to insist on any preconditions it likes for the triggering of Article 50. "
Then it also follows, jim, that Parliament is free to halt the process of leaving altogether. Because the alternative to no satisfactory leaving deal being agreed is not to leave. Whilst I agree that, following the SC ruling, that is perfectly possible, is that really what MPs are seeking to do? I wish those that are seeking such an outcome would come out and say so. All we get is "We don't want to overturn the result of the referendum but...." These are meaningless platitudes. If MPs feeling this way told the truth (which would be, for some of them anyway, quite a novelty) the electorate would know where it stood. As I keep on saying, the electorate wasn't asked "Do you want to leave the EU provided we get a good deal/provided we stay in the Single Market/provided it doesn't cost too much". They were also not asked if they wanted to half leave/leave the bits they don't like/retain the bits they do. They were asked whether they wanted to leave. They were told in a pamphlet issued by the government that their decision would be enacted. They should not have to concern themselves with the niceties of constitutional law when asked such a simple question and been given such a straightforward assurance.