A bit of a debate has been running on Twitter about the extent to which the vote to leave the European Union was driven by immigration. It is unresolved: most of the evidence suggests that immigration was a key factor, but far from the only one, in producing a small majority in favour of Brexit.

Certainly, “taking back control” was interpreted by many as reducing the flow of foreigners. It was a pivotal moment during the campaign when the Office for National Statistics released figures showing that net migration — long-term inflows minus long-term outflows — in 2015 had hit a record of 333,000.

As a vote influencer, it appeared to rank up there with Harold Wilson and the bad trade figures caused by the