Well, on the basis of 1 chance in 5, which seems to be roughly right, and assuming they don't bias the odds if you've done it last year, then 1 in 125. But that's the odds beforehand. Once you've already done two the odds on the third one is still 1 in 5.
However, I expect they do bias it, but I don't know how. If anyoen knows I'd be interested.
