It has been a busy day in Brussels, London and Glasgow on Brexit business.

In Brussels the British Ambassador to the EU, Sir Tim Barrow, hand delivered a letter from Prime Minister Theresa May. The formal purpose of the letter is to fulfil the requirement under Article 50 of the Treaty on European Union that a member state wishing to leave the EU must give notice of its intention to do so.

But it was the most open of open letters, intended for a global audience including EU neighbours and for the British electorate.

It could not entirely escape the influence of Government lawyers - it is after all the beginning of a process that is as much legal as it is political. The statement that "references in this letter to the European Union should therefore be taken to include a reference to the European Atomic Energy Community" was needed to make it clear that ‎we are leaving Euratom as well as the EU but slightly jarred with the rest of the letter and its focus on high political principles.

While delivered to Mr Tusk there is no doubt that the letter was addressed to UK voters and to the devolved administrations. Article 50 did not demand a six page letter which explained, amongst other things, that "From the start and throughout the discussions, we will negotiate as one United Kingdom, taking due account ‎of the specific interests of every nation and region of the UK as we do so." That statement was added to by other commitments to consultation with the devolved administrations on the return of powers to the UK. These elements no doubt included by way of part response to the resolution of the Scottish Parliament yesterday authorising the Scottish Government to seek powers from Westminster to hold a second independence referendum.

In London the Prime Minister addressed the UK Parliament, describing the delivery of the article 50 notice as a moment from which there is no turning back. Not a question for today but note whether the notice is 'revocable' remains a question on which lawyers are divided [hyperlink to House of Lords report.