According to Bojo if we leave with no deal then we won't pay a penny.
I think it's right that we pay what is strictly legally due, but I'd expect that to be fraction of the £39bn.
For example, about half of the £39bn was our contribution for the so called transition/implementation period from 30th March 2019 to 31st December 2020, even though the only difference practically was that we'd have no representation within the EU. No MEP's, no Commissioner, no Judge at the ECJ and no seat for the PM at the Council of Europe.
Other monies demanded by the EU and conceded by May/Robbins were money for projects that hadn't even begun yet (even though we'd given 2 years notice of withdrawal) and money upfront to cover loans that the UK had guaranteed at the European Investment Bank, even though there was no reason why we couldn't continue to guarantee those loans in the event of default!
I even think that the demands for cash to cover pensions is pretty spurious!