It turns out there is a simple solution to this problem, in the admin panel, just go to System -> Global Checkin ... and all the locked items will become unlocked!! The concept of locking anything which is web-based annoys me immensely, but at least I know how to work around it in this instance - hopefully I'll remember next time I have this problem.
I run the website for my local netball club and when I implemented this site I chose Joomla! to run it on. It seemed like a good idea at the time, in fact I even thought some of the other club members might log in and type into the user-friendly WYSIWYG editor to make content appear. Well, it doesn't happen often - and when it does it seems like Joomla! locks up all sorts of things, from articles to menu items, which means I can't edit them!