not quite correct therugratref, only 25% of places are determined by the test scores, the rest are decided by distance (after looked-after children and medical/social need)
there are plenty of local children in extension who didn't 'pass' the test, and several candidates from outside the area who scored higher who gained a place but are in upper, not extension - the test scores are one element in the setting decisions, but not the only one - predicted and actual SATS scores plus current school recommendation all count as well. The school also does their own cognitive tests early in Y7. Children getting all level 5s at the end of Y6 could be pretty confident of being in extension. There is some movement between the sets and as children go further up the school they may be taught in an extension class for maths, but upper for English, for example
There is still a sibling rule, but it now only affects siblings of children who got in by distance not test scores. This means there are plenty of families parachuting in from Balham etc and renting somewhere on the doorstep to get their eldest child in, then moving back to their 'proper' house and watching all their other children get in.
The classes in extension are huge - usually 34-35, not much smaller in upper, support is usually 15 or so, don't know much about middle. The sets to be in are extension and upper - some kids we know prefer to be in upper as there's slightly less pressure, but it's still v academic. Behaviour in 'middle' is said to be problematic, but in the higher sets it's very good. Because it's a big school, although there are opportunities on offer, not everyone gets to take advantage of them - e.g. not every Y7 goes on the PGL trip.
The catchment area used to be truly miniscule but since the abolition of the test siblings rule and the reduction of the number of selective places it has got wider, so kids who a few years ago wouldn't have got in now do so comfortably.