Ok, I have now listened to the first part of question time again. The first speaker, Sajid Javid, immediately changed the question from whether it was legally binding to whether it was morally binding, and the second didn't say anything about the legal side but that because of the result most mp would vote in accordance with the result. Javid also said that when MPs voted for it most thought it was binding but left vague whether he meant legally or morally.

Which leaves us in the same position, I think. MPs may consider themselves bound for reasons of personal morality and conviction. That is quite a different thing to whether they are bound in law.