Page Layout - Not sure how this has happened

[DawnG]

I've re-designed the banner for my website www.onlineheadshop.co.uk/catalog and now I've got a white margin appearing at the right-hand side of the centered area. I have Basic Design Pack installed, I've double checked the pixel setting in the Style Sheet, and have adjusted the width of the image accordingly - but it is still off.

 

What have I done? I know that it has got to be something simple, but I cant for the life of me figure it out.