Sally-Anne, do you think that I should worry about the fact that the letter didn't arrive today?
No, I'm afraid this is quite a common occurrence - letters posted on a Friday first class not arriving until after the weekend. I would like to think that the school posted all the letters on time, so the fault probably lies with the Royal Mail.
I hope the wait doesn't spoil your weekend too much.
If you don't receive the letter on Tuesday, call the school. They may give you the information over the phone, or they may insist on sending you a new letter. They might also be willing to tell you where he is on the waiting list if he didn't get a place. Although most 12+ places are taken up, there are occasionally people who decide aginst disrupting the child's education, or children who just decide they don't want to move school after all.