Having asked around, I'd venture the hypothesis that IMO contestants account for about 1 in 10-12 Cambridge entrants, and about 1 in 4 Trinity entrants.
So in 2009 Cambridge admitted about 20-25 IMOers, and Trinity about 10.
I'm quite surprised by these figures, but this is the picture that's emerging, based on bits of info that various people have supplied. If anyone thinks the figures are wildly wrong, please let me know.
If they are reasonably accurate, is this a new thing, i.e. for the 2009 intake? Or has it been like this for a decade, or what?