PPL152,
As far as I am aware, bookings are done through the website not via email (if I am wrong then just ignore this next bit!)
You can log on using your application number (If you don't know it then it will be in one of the emails they sent you when you passed Phase 1) and the password you chose when you originally applied. Once you have logged in to your application, it should come up with all of the available Phase 2 sessions, and you just click on the one you want to attend then give them a call and pay for it.