///Cameron only promised a referendum to prevent a landslide UKIP vote, because they had threatened the very same. ///
Yes, togo, and the last thing he expected was having to honour that pledge. If he didn't lose the election, not having a referendum would have been blamed on his coalition partners. Imagine his initial euphoria on election night draining away with the grim realisation that he had an unexpected outright majority and could no longer blame the Liberals for all his weaknesses and failures.