Stone the crows! I thought I'd return a few days later and see if anyone had come up with an answer to those three questions and by the timings on the posts it looks like I made the usual mistake of judging everybody else by my own low standards.
Chalks 7th Law of Teaching states that the intelligence of the staff is proportional to the intelligence of the pupils in their school. So clearly Mark Wilkinson does not work at St. Thickchilds. He wins because he was first to get them all right and even spotted my foolish mistake of assuming that the rule was proved after just four measly examples.
Oh I forgot; number 3 is the Monty Hall problem. I just tried to rewrite it slightly but as anonymous has pointed out it only works if I know which cup the diamond is under, so I can always choose to show you the other, empty one.
I do quite like problems like these as they demonstrate just how wrong common sense can be in unfamiliar circumstances. Mind you, I like beer more.
I'll have to think up a suitable prize now. Suggestions welcome