We were on the waiting list for Colchester Girls last year.
DD was ranked 151 and didn't get in on allocation day. A week later we rang the LEA and was told she was 7th on the list. To give you some idea of movement, she went from 7th to 4th in two weeks and then 4th to 1st to being offered a place within another week.
My understanding from talking to the LEA folks last year, is that there is quite a bit of movement before the date where you have to confirm/reject your place. Last year I think this had to be done by the 3rd week of March, and even after that there is still some movement. My DD knows of quite a few girls at CCHS (Col) that were higher up the waiting list and were offered a place.
I also understand that you can write to the schools and ask to be kept on the waiting list even after the first term, then should a place become available all children on the waiting list are invited to sit another exam and the person with the best marks gets offered the place.
I can also confirm Notley Parents comments regarding very very late offers. One DD tutored by my DS's tutor two years ago got offered WHSG, got the uniform and then in the summer holidays was offered a place at CCHS which was then accepted, thus releasing a place at WHSG - so don't give up hope.
I agree with GibsonES335, it's certainly a bumpy ride....