Thinking back there were a few problems. One of them, was that our school had not done anything like this before and had not taken time to read the instructions. They offered to do both tests, one after the other, after school
The room he was given was not at all quiet. I sat along the corridor, in case a 'runner' was needed - luckily I did because in that 50 mins, 3 teachers tried to get into the room because it was normally used for something else. The noise from the children around the school was also very apparant. Also, make sure they have scrap paper - it wont be provided.
On the plus side, it was nice not having the frenzy of the playground beforehand and within reason, choosing a time to suit us, however, he had not visited any of these schools and therefore could not really appreciate just how important the exam was. I do remember receiving the results about a week after everyone else and that wait was awful, but Bucks CC cannot give results out over the phone and that is fair enough.
I am very glad we did it but really felt on my own a lot of the time - there was no one to discuss opinions with etc or even understand what the 11+ meant, over here - and that is probably the same for you.
Anyway good luck with your preparations and please do come back with any questions - we would be happy to help.