Thank you everyone for your replies and I'm genuinely pleased to hear that there have been good experiences.
In my form, about a third had an assisted place (economically crumbling Northern town). Everyone knew who we were. I remember during my first week a girl asking me whether I was on an assisted place. When I said (proudly!) that I was, she pulled a face and physically recoiled. I still remember the realisation dawning - this thing that I was so proud of was somehow...dirty. To be clear, that was not the attitude of all, and even she softened as she grew and began the adolescent process of critically appraising her parents' views. But there was an undercurrent of this attitude in the school. I think some of the teachers held it too. I certainly felt that some did not expect much of me.
Any maybe we were an unusual cohort, but as a group we really didn't shine academically. This was an era of class rankings and we reliably and disproportionately took the lower placings. One girl did do extremely well and is now a vet, but she was the exception. Around half left after GCSEs having got pretty mediocre results. The brightest girl, let's call her Karen, was a bit of a phenomenon. Even at 11 I could tell that she was on another level. She had a razor sharp intellect, and a memory like a mousetrap. She should have soared academically. Instead she got a reasonable but not stellar bag of GCSEs and A levels and now works as cabin crew, where in fairness, her flair for languages comes in massively useful and she is very happy! Another is in prison! Another, again very bright, left at 16 to go to catering college, and has since dragged herself through evening classes and Open University to achieve a career that she should have managed with her eyes shut 25 years ago.
If anyone is interested, the research that I read that triggered me thinking about this can is summarised here:
theconversation.com/the-state-has-helped-poor-pupils-into-private-schools-before-did-it-work-70222
In short, it seems to suggest that the scheme largely failed in its aim of attracting genuinely disadvantaged students. But, where it did find these children, it probably did them more harm than good. The middle class children who accessed the scheme (of whom there were very many) tended to do rather well.
I think this story fits what I saw. My school, it turns out, was one of the successful ones in terms of attracting genuinely poor children into the scheme. However, as shown in the research, it did not manage to convert this into good outcomes for those children.
I'm not sure where this leaves me. I love the idea of private schools offering bursaries to bright disadvantaged children, but if this is going to decrease life chances for them.... Tricky.