"Actually what it also asks is how much success is really down to the school and how much is down to the pupils they manage to cherry pick"
I think it's always very much down to how selective it is on entry. If you are going for the top 5% of pupils, you'll get stellar results at 16 and 18. And there are enough children in London to sustain several such schools.
Emanuel's results will rise, no doubt about it, because of demographics (more children) mean that the pass mark for newcomers to the school is rising. But it is committed to a very inclusive sibling policy (they just have to reach the minimum pass mark, and can have one resit) and they claim to attach importance to the interview (yes, lengthy individual interviews for all 600, which is the number where they stop accepting applications, and why they have to guillotine) as well as to the actual exam mark. So they aren't aiming solely for the most academic pupils, meaning that there always will be a spread of achievement.
The headmaster doesn't seem really to care that the school isn't going to be the top of the league tables (you can't opt out of the Dept of Ed data collection, so they do appear in some of them).
Yes, children really are turned down these days. And as there are two year 6 classes at the moment there will probably be fewer year 7 places up for grabs this year, as I don't think they're planning to increase the numbers in the main body of the school (if anyone's going to an Open Day or other visit, might be worth asking to find out exactly what the official line is).