There's is a page on it inthis weeks cycling weekly
It basically said it is downto the size of the pedal ie smaller on the ball of foot can be caused by pressure but not likely shoes and don't move cleats too far can hurt knees
sorry, ive realised what ive put. yeh tight shoes and straps caused problem, loosen them then alls fine. whats the point of straps if they are so loose they barely work?