I thought you would have known the legal status of article 50 Eric, most disappointed.
If there is a NO DEAL outcome - and the time duration is up - then in purely legal terms - we owe nothing BUT that would also mean we would have walked away - so there would be no deal on other things as well.
There is the grey issue of article 70 of the Vienna convention - that is to do with international treaties. So the EU may argue this case, but from what I have read the Article 50 treaty would trump the article 70 treaty (I think) because of the EU treaty itself already caters for this - with Article 50 - so it would be a weak arguing point from the EU.
And as we all know Eric - Article 50 does not say anything about money or rights or obligations. Article 50 says "the treaties shall cease to apply to the state in question" either when a withdrawal agreement takes effect, or two years after the Article 50 process has been triggered by the member state that intends to leave.
So in legal theory alone - walk away means walk away. But as we all know - we have things they want - and they have things we want. So a business driven compromise will be found. Politically fudged agreements with be put together in the 2 years post 2019 - I think anyway. With the main core agreement in place for 2019.
2018- to 2021 is a very long time in politics , we could have Prime Minister Blair II , Prime Minister Corbyn (
) , another Conservative PM. I dont think / hope there will be another General Election before 2021. When the new PM could offer a second referendum. I forgot to add in the vote in Parliament at the end of Article 50 - where MPs get to vote on accepting what is on offer - or walk away - at least I think that's what they are going to vote on.