as far as I know you have to apply again and you get put on the list same as everyone else. what I did was on the section of the form where it says put any other information (twas a couple of years ago now so can't remenber exactly) write down that you would be willing to accept a cancellation at short notice. I did this and had my test in about a month. As I said that was a couple of years ago so they might have changed things since, or maybe I was just plain lucky
hope this helps
Edit:that was my first time doing the test, not a retest