The government have already got the weapon that they require. The New Fire Services Act gains royal assent next month funnily enough around the same time that the government said it requires to train the forces.
For the government, a 'Yes' vote in the ballot is a double gift. First prize is that Prescott could use his retained powers under section 28 of the act to impose a settlement and 'control' the union. Equally valuable is that he could then say - "The NJC can't negotiate, we'll impose a pay board (like teachers) telling you what any pay increases will be, and it's obvious the fire authorities can't manage the service either, so we'll regionalise everything" At one stroke, he gets rid of several knotty problems in good time before the election.
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