Viewing the page source, I can see that it's requesting loads of CSS files for styles_debug.php. e.g.:
<link rel="stylesheet" type="text/css" href="http://website/theme/styles_debug.php?theme=clean&type=plugin&subtype=mod_book" />
There are about 30 of these, which obviously slows the page loading time considerably.
Have I changed a setting somewhere to do this for development purposes? How can I change it back?