Indiemum wrote:
Thank you for the reply. It is so confusing - I have been trying to read up on it but I still don't get it!

There seems to be a "pre-test" which is multiple choice online in year 6 and also a common entrance exam in year 8 and conditional and unconditional offers - every school website seems to say something a bit different.
Both my DS went to prep school but left at the end of year 6 to go to GS. They were in the minority but not alone. For those that remained many sat pre-tests as you describe and then had to get the required grade in the CE. However, things may have changed and even then every school seemed to be different. Both of my sons has boys join their classes in year 5 or 6, it's quite normal so your DS probably wouldn't be the only new boy.
Why not identify the schools that you are interested in and look at their websites. Or, have you identified a prep school that you like? Make an appointment and go and see the Head, they will be very used to this situation and will be able to explain the process for the schools in your area.