General election: Do Conservatives claims about Labour's spending plans add up?

Conservatives have taken a leaf out of the Vote Leave handbook by putting a Very Big But Questionable Figure (VBBQF) at the heart of their campaign efforts over the past few days.

For Vote Leave, of course the VBBQF was £350 million - the supposed amount handed over by the UK to the EU each week - which was plastered over Boris Johnson’s battlebus and mentioned by campaigners on every possible opportunity despite being ruled “misleading” by the official statistics watchdog.

For Tories in this election, the figure is £1.2 trillion - the additional spending which they claim Labour is committed to over the next five years, despite the fact that Jeremy Corbyn’s party has not yet agreed its manifesto or set out its plans.

Although this VBBQF was rejected as “ludicrous” by Labour and questioned by many observers after being dropped in sympathetic newspapers on Sunday, Conservatives have followed up today with a claim that the supposed extra spending would cost the average worker a month’s wages each year in higher taxes.

So how was the figure calculated, how reliable is it and what is the tactical thinking behind its use?