Speaking to a meeting organised by Edinburgh West SNP last week, people were surprised when I told them my grandparents lived for many decades in West Pilton Gardens. They were surprised because I look and sound like an English ex-public schoolboy. I appear like this because I attended an English Grammar School, the entire purpose of which was to turn out ersatz public schoolboys. So am I walking proof of Theresa May’s thesis of grammar schools being the answer to social mobility for bright people from poor backgrounds? No.

The wrench has stayed with me for 46 years of my little group of primary school friends being split up between those who were sent off to Paston Grammar School and those who went to the local secondary modern. With the best will in the world, the separation became insuperable very quickly. I am back in Sheringham now for my mother’s funeral. It is very plain that an exam at 11, which you could pass or fail by one mark, changed people’s lives forever. Simply put, the majority of people still around are those who failed the 11 plus, as a pass led on to university and much wider life opportunities. Anyone who tells you that secondary moderns do not carry a stigma is lying – and you cannot create grammar schools without creating secondary moderns, whatever you call them.

Socially it was still worse as two of us four siblings passed the 11 plus and two failed. There is no doubt at all that this exam result at 11 – which had nothing to do with any difference in intelligence or industry in the family – irrevocably affected our careers and even, to some extent, the nature of our family relationships, though we are still very close. What is more it is not a coincidence that the two who passed were the eldest two – and we both had started our primary education when the family was very well off. By the time the younger two started we had fallen on hard times in a big way, and had to move. There are numerous statistics to prove that selection favours the wealthier. I know this.

The grammar school itself was absolutely modelled on the lines of a public (which is Orwellian for private) school and I believe had been one. It had this very strange militaristic ethos, and the teachers were all men who had been profoundly affected by fighting the second world war. There was a Cadet Force where you had to dress in real military uniform once a week and fire guns and march up and down a lot, and it was plain the aim was to turn you from a rustic youth into a member of the officer class. Latin was compulsory, discipline was extreme, and teachers thought it amusing to throw blackboard rubbers – with heavy wooden backs – at children’s heads.

It was so successful in turning out ersatz privately educated pupils that I have been mistaken for one more or less since. And there is no doubt at all that this helped me get in to the fast stream of the FCO – in an intake in which I was one of only two state school educated entrants in the fast stream. There were two graduate entry streams – administrative (fast stream) and executive (slow stream). In 1984 there were just two state school entrants in the fast stream, and no private school entrants in the slow stream.

It is this plucking of hearty young yeomen and turning them into officers for which Theresa May nostalgically yearns. But I absolutely hated the school. I hated the discipline, I hated the militarism, I hated the narrow thought. I hated it so much I performed terribly – I got a B and two E’s at A Level and scraped into university on clearing. Yet once in University with much more personal freedom, I flourished and never in my entire University career came less than top in any exam I took, culminating in a first class degree. The grammar school system had almost destroyed my potential because of my reaction against its class divisiveness.

Once in the FCO, I could perhaps pass in manners, knowledge and speech patterns as one of my fellow high fliers, but thankfully I lacked their class solidarity and social codes. That is why I could be a maverick and a whistleblower, and not go along with torture and extraordinary rendition, with which the Flashmans who dominate the FCO had no problem.

Theresa May was absolutely right that there already is selection in education, and that it is selection by wealth. But those buying a private education are not actually buying a better education – they are buying admission to a social network of wealth and privilege, bonded by common contacts and attitudes. The answer is not to sneak a few people into these networks, but to dismantle the social structures that have beset the UK for generations.

Bringing back grammar schools will not increase social mobility. Abolishing private schools will increase social mobility. The State can and should insist that every child has a state education, secular and of the highest quality. Attendance at secular state school during normal terms and school hours should be compulsory for all children in Britain.