£350 million for the NHS when we leave the EU. This claim is in the news again. But is it credible?

£350 million a week extra for the NHS. That was the controversial claim from Brexiteers leading up to the EU referendum last year. It's back in the news this week after a huge row started between Boris Johnson and the most senior civil servant responsible for official statistics. Sir David Norgrove said he was "disappointed" the foreign secretary had revived Vote Leave's pledge of £350m a week extra for the health service.