I'm going to do a quick thread attempting to summarise the Supreme Court's findings, and its reasoning, in a way that is accessible to non-lawyers.
The court first asked itself: is the PM's decision to advise the Queen to prorogue Parliament justiciable? That means: can the court actually rule on whether it is lawful, at all?
The court said it is justiciable. The *extent* of an executive power - what the power authorises a minister to do, and what it doesn't authorise - is *always* a question of law.
Still on justiciability, the court notes that Parliament is sovereign. That would be undermined if the PM could prevent Parliament from exercising its legislative authority for as long as he pleased.
The court then goes on to an extremely striking discussion of another principle: "parliamentary accountability". Part of Parliament's role is that MPs ask questions, look at delegated legislation, scrutinse. All this prevents the arbitrary exercise of executive power
On the substance, the court therefore makes this central conclusion. This is the very heart of the judgment: 
This prorogation frustrated Parlt's ability to carry out its constitutional role, said the court. It was 5 weeks: Parliament might have voted not to go into recess, given the circs, and so might have wanted to sit for much of that. - That alone might not make it unlawful. But..
The circumstances are exceptional. A fundamental change is due to take place on 31 October. "Parliament, and in particular the House of Commons as the democratically elected representatives of the people, has a right to have a voice in how that change comes about."
The court asks whether the PM has a reasonable justification for such a prorogation. HE doesn't, the court says, because if he just wanted a Queen's Speech, he did not need to prorogue for 5 weeks. Sir John Major made clear that such a long time is not needed to prepare one.
So, the court says, Parliament was never prorogued. To make this finding does not breach Article 9 of the Bill of Rights, which says that proceedings of Parliament shall not be questioned in court. Prorogation is not a decision made BY Parliament, but something done TO it.
That is what their lordships said. Will put something out later on what I think it all means for the constitution.
/ends
/ends
