Thanks for the kind welcome everyone.
Yes, Mark, I've looked on the floor, in every conceivable gap - even moved the bookcase! Still no Betta. I figured he jumped out through the small gap where the cords go out of the tank, under the hood - even pulled my filter apart to see if he fell in the finger-hole where you lift off the lid (I have a Juwel tank). Suppose he will turn up somewhere really weird :roll: