Thanks, that seems to do the trick.
Had to readjust the width of $CKEditor_config["width"] to compensate for the change of font and size.
The editor window now shows the same width and fonts, background colour etc. as the rendered site.
The only drawback is that it introduces a horizontal scroll bar in the editor window but appart from that, it is a true repesentation of the site page and will be much easier for the client to create their own content as its WYSIWYG.