Oh gosh, please don't bank of the results of the Sutton Mocks! Had two friends DS's who got really high scores and neither even passed, Sutton, Wilson's or Wallingtons let alone get offered a place (note, often people pass but still don't get a place. Four hundred may pass, but there are only 120 or so places)! The mocks don't include a writing test and the actual exams do, which might have some bearing as to the reason why. The school will warn you too, it's just to get the kids used to an exam environment. They are run by the PTA, so even if you ring up late, you can still get a spot as the money goes towards their kitty. I think a friend of mine got a space from calling just a few days before. Like @tiggytape said re competition, the same applies to Wilsons and Wallingtons - it's firece; and each year the numbers rise and the kids get more clever. But who knows, he could be one of them, it's not impossible:).
Archbishop Tennsion have a great ofstead report, the kids are nice and if they are active in the church then they shouldn't have problem getting a reference from their priest. A catchment area also applies though. Not sure about being musical either, their specialism is Maths.
Please do not rule out Whitgift and Trinity as their bursaries are so generous and on a sliding scale for people earning up to around 70k'ish (Can't remember the exact figure). What a lot of people fail to take on board however, is not EVERYONE gets a bursary even if they're eligible and even if they get offered a place. I think it's something like the top scoring 70, or something like that.
Have you looked at the feeder school, or at least the schools most of the kids go onto? Are they that tragic?
I think it's just good to remember you have six preferences, not six choices and therefore to have as many back up plans as you can. It is a stressful time, there's no doubt about it. On a positive note, most get one of the schools on their list, which may not sound positive, but it is. For those who don't, they get offered the schools that nobody else wanted and that can be heartbreaking.