Well, posting on here seems to have magical properties as after an hour or getting nowhere, the menu is behaving itself again. This is not the first time I've had a problem, though never for more than some minutes, does anyone have any idea what may be causing it?
Plus, if I could make a suggestion for the number one most helpful improvement WordPress.com could make (and I remain a fan not a critic!) it would be to have a better way of managing menus, so you could select the order by number in the menu area (not on each page) and assign top level and sub-menu items by number not drag and drop, a bit like the widget system in Twenty ten. Thank you.