"Strolling into one of the rooms of a London club, I noticed a position left by two players who had gone. This position is shown in the diagram. It is evident that White has checkmated Black. But how did he do it? That is the puzzle."
You're missing a move. How did the Bishop get there to force the pawn to block? The en passant capture is correct, I'm just confused as to how the bishop got to b1 to give check.
Came across this in my internet stumbling.
"Strolling into one of the rooms of a London club, I noticed a position left by two players who had gone. This position is shown in the diagram. It is evident that White has checkmated Black. But how did he do it? That is the puzzle."
From:
http://www.futilitycloset.com/2007/03/15/chess-detective-work-2/