Sate wrote:
I have looked at their websites but it's all very confusing. Say we are keen to apply for Wilson's and Sutton Grammar School, do we then register online at both of those school websites or do we apply to the Sutton Council for the exam?

You only register for the State 1 test (the SET) once. You can do it via any of the participating schools' websites. Your son will then have to sit the SET at one of the Sutton schools (you will be told which one in an email with all the arrangements) and again it doesn't matter which school you sit the SET at, the score will apply for all the schools in Sutton.
Wilsons and Sutton then use the SET to filter for a second stage test whereas Wallington Boys and Greenshaw just use the SET.