Hi, sorry for the delayed response!
The head PHP did have unclosed div and body tags! Sadly, this didn’t effect it.
The validator did came up with plenty of problems (20) but none of them were as you described.
Which style elements do you think could be the culprit?
Its a modified Kubrick (WP default) template and there are so many tags I suspect I’ve changed one little thing and forgotten which one it was.
Any ideas…?