Thanks for the usual quick reply. The problem applies to several different users and it only registered with me when my database backup failed - due to the char_length of the field on which the html editor is applied (over one million characters seems to cause the issue).
Once the guff was removed (the page displayed fine with guff in place) there were only a few hundred characters in use.
I have just tried uploading a png, and the displayed code is what you would expect!
Could it be a virus on the users' computers?
(Thanks for IE10 news - will wait until it's available)