Well done Bouncy. I've never won anything from the premium bonds either but I did win £50 at the casino the other day (I only went in, in passing, to see a friend who, it turned out, wasn't there that day).
The difference between fiction and reality? Fiction has to make sense.
They draw the higher prizes first, so any envelope that arrives at the begining of the month is going to be better than one that arrives half way through the month.