Ian, I'm using firefox ver 18.0.1 and your site has no nav menu on the side. All I see is an image and then a centered strip that 610 px wide that contain words and images until you get to the words
These are key skills that stay with our students for life whatever paths they
at that point the strip goes to 444px by sucking in the right side.
Think you lost your css styling.
OK I found your problem. Stop using position:fixed; position: relative; or position: absolute; Use the normal flow of html and I think you'll solve all your problems.
BTW the nav was under the strip because you used position: fixed; on it. I have a 1440px wide screen and mine is small.