Thank-you for the reply!
I've tried replacing images in the images and content_server folders. It has no effect.
Some limited success has been achieved by editing the html files in the content_server folder and substituting a link...
http://calibre-ebook.com/favicon.ico
...in the index.html and browse.html files but I haven't quite got it to work. I'm not too familiar with html and creating a website icon.
I must be missing something else.