OK made a few more tweaks:
Reduced the width of video in the forum – ideally will later have a proper solution which allows them to resize with the content area.
The image resizing problem on mobile Safari has been tied into the site not scaling down correctly. I’ve made some progress on this, it’s still not completely there but it’s better than it was. Feedback from anyone using mobile iOS devices would be appreciated.
Stopped the ad moving around in the header (thanks @arowan)
Fixed incorrect colouring in some menus
Cosmetic improvements including making the forum titles less bold
Thanks for the feedback and tips so far everyone, keep ‘em coming!