If they call a general election because they lost a referendum on AV, that will be perceived as sour grapes and not accepting the will of the electorate.  Thus even if they got 40-% of the vote for AV in a referendum, they might get less than 20% in a subsequent General election.  Failing to accept a referendum result is the very worst reason for calling a General.  They would have to find another legitimate reason to pull a general...

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