Tracked down the problem. It was the doctype thingy
If we have this at the top of the HTML then the full screen mode seems to think the browser window is bigger than it actually is.
The obvious solution is not to have this, but our web designers are not happy. They mentioned something about accessability and the more immediate problem when its removed is that all the styles go to pot.
Does version 6 fix this bug?
Is version 6 still appearing at the end of the month?