Please help. My header is really doing my head in now.

It’s lining up with the style sheet size just fine.

Have you noticed this image as the background for the page?

http://steel-city.co.uk/blog/wp-content/themes/default/images/kubrickbgwide.jpg

You’ll see that the grey is not the marker of the margins of the page.

You’ll have to either increase your page size, or decrease your header image size by however many pixels the grey space takes up.

Trial and error is probably the easiest way to do that.