Hmm... I haven't modified the plugin, other than adding the pro files as per http://www.simpleviewer.net/simpleviewe … iewer/#pro and went from there.
The host would have installed the mod_pagespeed module whilst testing speeds. I added '?ModPagespeed=off' to the end of the URL to disable it and that seems to have done the trick. Once again, it was the hosting's doing. I'll uninstall it as it doesn't seem to be affecting the speed at all.
Thanks for your help.