When Theresa May first stood on the steps of 10 Downing Street, few realised that her pledge to 'help anybody, whatever your background, to go as far as your talents will take you' meant selective education was back on the agenda.

But, as a civil servant accidently confirmed this week when photographers snapped a briefing paper he was carrying through the same doorway, the government does indeed want to open more grammar schools.

It is a debate where anecdote often competes with evidence.

On one side, those who attended grammar schools talk about the difference the system made for them. On the other, research like that from the Sutton Trust in 2008 has found grammar schools take 'half as many academically able children from disadvantaged backgrounds as they could do'.

The first involves successful individuals who are easy to relate to; the latter relies on abstract statistics that are harder to engage with.

The government itself recognises it must address concerns that the selective system can be bad for those who are left out. The leaked memo said it needs to work with existing grammars 'to show how they can be expanded and reformed in ways which avoid disadvantaging those who don't get in'.

Much of the debate will focus on whether grammar schools are good for social mobility, but it is also worth considering the political calculations behind a policy change that goes against the Govian thinking that has driven government education policy since 2010.

For Mrs May, a Remain supporter who now leads a party full of Brexiteers, this is an issue where she can align herself with the instincts of the Tory grassroots, and differentiate herself from the distrusted modernisers who preceded her.

Not insignificantly, UKIP is a pro-grammar school party, and is now the main challenger in many traditionally Labour areas in the north.

If the prime minister can frame her policy as fighting for hard-working families, she may see it as a chance to woo the Brexit voters and disillusioned Labour supporters she needs to win her party a solid majority in 2020.