I am surprised by the number of people who think that once the result of the referendum result is known, it will then be up to Parliament to pass an Act, if it chooses to do so, to introduce the Alternative Vote (AV) in the event of a Yes vote. A professor at the recent conference in Leeds thought it would be. The same point was made on the Today programme earlier this week.
The referendum is not an advisory referendum. The Parliamentary Voting System and Constituencies Act 2011 (section 8) provides that, in the event of a Yes vote, AV will be implemented by order. No further legislation is required: Parliament has already stipulated what will happen consequent to the referendum: if there is a Yes vote, AV will be introduced for parliamentary elections and if there is a No vote it will not be introduced.