Thanks Sarah. I did have a look around online at vids on teaching this trick (and came back to try to delete the post, but couldn't see a way to do that.) I was holding the treat just above his nose. From a couple of different vids, it looks as though it's okay for the dog to lean on your hand or even to brace him with a hand on his chest so he learns to hold the sit in an upright position. He's sleeping peacefully right now, but back to the drawing board tomorrow.