The hearing is fascinating, I can now point out what I think it will hinge on. It is quite a Gordian knot for the judges to undo.
The appeal is contending that the Prime minister, acted improperly in prorouging, because his motive was to prevent (stymie) Parliament so as to avoid the risk that parliament could interfere, or stop his free reign to enact his policies.
The argument for the defence is essentially that the prime minister has free reign to proroge for whatever purpose, for as long as he likes, and whenever he wants. And that the courts can't stray into his powers to do it, because it is a political act and would breach the separation of powers.
There are serious flaws in the argument for the defence in my opinion,
Firstly, it is well established in law that no one is above the law, so if the Prime minister acts unlawfully, he can be sanctioned by the court.
Secondly, it is established that the executive(government), is accountable to parliament, as sovereignty rests in parliament and not the executive. So if the exectitive silences parliament to continue on a course for which it does not have parliamentary consent, this relationship becomes reversed. The tail is wagging the dog, rendering the constitution broken. So the Supreme Court must sanction the Prime minister so as to maintain, or protect the constitution.
Third, if they rule in favour of the defence, the government, then the prime minister will have free reign to proroge again on the 14th of October ( when parliament is due to return) until after the 31st of October, enabling the Prime minister to take the UK out of The EU with no deal, simply by inaction and parliament and the Queen would be powerless to stop him. Well I think the Queen could refuse him, but it would compromise her impartiality. So the Supreme Court must sanction the Prime minister so as to protect the Queen and therefore the Crown.
Fourth, I think it can be argued that sanctioning the Prime minister for certain conduct is not actually becoming embroiled in politics, but rather protecting the systems and working of parliament, so as to maintain parliamentary sovereignty. And the proper working of the constitution.
So I can't see how the Supreme Court could do anything other than to find the Prime minister to have acted unlawfully and sanction his powers.
Im looking forward to the intervention of John Major tomorrow.