Thanks for all the "condolences" on our SP failure. It was always more than aspirational for us and we did not plan to apply initially and so weren't that disappointed, and in fact, a bit surprised he got through to the final stage.
But for parents planning 11+ ahead and reading this, there are certain observations I can share, in particular on the interview. It would probably be most relevant to the most selective schools (perhaps Westminster, Eton, and St Paul for boys) which are usually the first choice for those that apply. It hasn't come to our mind until the interview date, when we saw quite a different mix of boys compared to what we saw in the stage 2 exam and on the open day, and more obvious after meeting the HM in the parents' interview when he explained the school's ethos. There were just too many of my boys' type. There were some 100 boys in the last stage, and with 36 spaces, maybe they offer 40-50 and a few more on the waitlist. Just some hypothetical number, maybe 10 or so are true geniuses they will certainly offer, but I imagine the exam score will be closer and closer when the ranking goes down to 100, and can easily be different if you gave them another set of exam papers. I think they then look for a mix they want to see to fit in with their ethos with the small difference in the score not really relevant, and for us, within the type of boys my son belongs to, he is at or near the bottom in the score.
For those still planning, you really need to make sure your kid stands out not just in exam scores but in something else that the school wants (and not everyone has).
Although we did not get an offer or wait list (they call reserve list), we do understand why the school choose boys this way, as without the ethos, it would not be the school it always has been.