Their admission rules should be the same as other schools in the area but they seem to get an amazing amount of high flyers who can't ALL live close to the school. They do fudge the admissions statistics by including the boarding numbers when they feel like it. They also have an abnormally high proportion of siblings which I haven't quite worked out why and which means a few hundred children scrabbling for the few places left.
Consider day-boarding? Apparently a way in. Sibs don't have to when their time comes.
Live next door to the school and go to a feeder school.
The aptitude test from when dd1 sat it was in a foreign language like Norwegian I think and there was a passage and questions. They didn't expect her to understand it but to intuitively 'know' what was being asked. If that makes sense.
Sorry not much help. None of mine went there so have no 'insider' knowledge! Good luck.