Paddy

Hold on! I've just found the answer.

The context menu was a red herring.

The folder in question has the "Customize" Property set to "Documents". That
means that when I click once on a document in Windows Explorer, it shows a
preview of the document on the right-hand side of the screen.

It seems that whatever program is being used to display the preview has put
a write-lock on the file. This means that I can "open with" another program
to read the file, but not to write back the changes.

I have set the "Customize" Property of the folder to "All Items". This has
allowed me to edit the file successfully.

Thank you for your time, and I hope that this post helps someone else.