Democracy means the right to change your mind. Collectively, people can decide to do something, and if it doesn't work, or they don't like it, they can do something else. That much shouldn't be in doubt for the left - or any democrat. There's no argument in principle against having a second referendum - on anything - if that is what people want.
Then again, you can't go changing your mind every week, or even year, or nothing would ever be settled. So, when is it right to have a second vote? I'd argue three things need to be considered. First, has the information on which the first decision was based changed or proven to be have been wrong? Second, have opinions changed - at least enough to suggest a different result? Finally, is the legislature charged with implementing the decision unwilling, or unable, to do so.