Thanks to everyone for taking the time to reply - much appreciated. Pleased to say my DS has already done the Highcrest banding test, so we're covered should we decide on that school. Sally-Anne - thanks for the allocation distances table - v. useful - based on the previous few years' allocations, we would definitely get into SWR, scrape into HGSS (we are in catchment but on the edge) and probably get into Highcrest (DS's banding score is on the Band A/B border, probably more likely Band B, in which case we would have got in there all previous years listed other than 2015). Of course, next year's allocations could be completely different ...
I've visited all 3 schools during the day (DS didn't want to be taken out of his primary school and he had already seen them on the open evenings, so I went on my own to the open mornings) - unfortunately none of them came out as a clear "winner" in my mind (but equally none came out as a clear "loser" either

). I would like to take the best points of all 3 schools, combine them into one school, and put it half a mile from our house

. If pressed for a preference, DS would go for Highcrest, but only because he could walk there, whereas he'd have to take a bus to the other two (or have a very long walk!).
I'll try to seek out some parents of children already at the schools and then we'll just have to make up our minds before 31st. (Of course, we may decide after much soul-searching that we want School A and put it first, only to find we are actually allocated School B...)
MrsChubbs - thanks for the thought about Chiltern Music Academy - we will certainly look into that as it sounds great!
