Just so you know the booklet they give you doesn't really tell you much about the assessment day or the process of recruitment you will under go, below is what it was like for me when I took my assessments and interviews back in 2008/09.
On the day of your assessments you will be in a group of around 5 people and you will start by having 3 written tests, which will be a reading/writing test (like the one from the booklet), the dials test (like the one from the booklet) and a communications test (basically listen to a tape recording twice and make notes about what is being said then pass on that message to the assessor getting the major points across). Once you've completed these tests you will be taken into another room for the computer tests, you start off by being logged in to the system and if I remember correctly you will do the dots test first then the reaction test. Before all that however you'll get chance to have a couple of practice tests first (my advice here is make sure the keyboard, headphones, pedals and monitor are all working correctly because once you start the tests you can't stop or retake them even if one of these bits start playing up, you will suffer for it i know of 2 people that failed this part because they didn't try out the equipment first they just jumped right into it thinking they were going to pass). Once completed you will be sent into the waiting room while your tests are scored and your results processed, you will then be called in individually to be told your results by your assessor and if successful you will be sent home and you will be sent a letter informing you of when you will have your interviews and if you don't meet the grade you will be told which part you failed on.
As I said above your Interviews (if you pass the assessments) will be carried out on a separate day (normally a week or so after you assessments), you will first be greeted by a member of recruitment and taken into a room where you will be greeted by a manager which can be either a depot manager or a Driver Standards manager (DSM for short) who may or may not be your future manager, I was going for a position at woking and was interviewed by the depot manager at strawberry hill so it could be anyone, it depends on which manager is on duty that day, both the manager and the member of recruitment will in turn ask you questions from there book and both write down what you answer and how you answer their questions. Once you have finished this interview you will be sent back in to the waiting room where you will wait to be called in by another member of recruitment and taken in to another room where you are given a booklet with some questions to answer and I think you get 5-10 minutes to fill out this booklet, after the time has expired you will then hand the booklet back to the assessor, this assessor will then read the booklet and ask you questions about what you have put. I believe this interview lasts about an hour and a half depending on your answers. Finally once you've completed both interviews you will be sent home where you have to wait for either a letter to come through the post or a phone call from recruitment to tell you how you did in the interviews and if your successful you'll be given a date for a medical which once passed you'll then have to wait for yet another letter but this last letter is the one you'll be screaming at the postman for because this letter will tell you if you passed the medical or not and if you did it will tell you your start date.
Hope that gives you a bit more of an insight into what to expect on the day a i said this is what I experienced back when i took mine in 2008 so should still be the same.
Good luck with it all,
Alan