Who decides on the final form of the ballot text?

Elections Act 1996

http://www.ecs.sc/resources/Rev29-election%20laws.pdf

Part IVSection 56

1. The question to be put to the voters at the referendum shall be determined by the Electoral Commissioner but shall be clear, precise and so framed that the voters are required to vote in the affirmative, if they approve the issue raised in the question or in the negative, if they do not.