#contents {
margin-left: 199px;
}

You can add this to the bottom of your CSS and play with the content width to see what you think:

.hentry .main {
width: 580px;
}

Thanks very much for your help, I'm new here and this forum seems brilliant for quick and helpful answers :)

Since widening the header means that you can no longer use the header uploader, it will not link back to the main site page like it does now. The new image would be put directly into the CSS instead.

There is a trick that can be used to do that that requires making a transparent image the size of the header (no background color) and saving it as a PNG image. You then put that image into a text widget in the bottom bar and link it to the site main page and then we would use positioning to move that image up and over the top of the header image.

The following will increase the overall width to 1000px from the original 800px. I've changed the width to 100% for the content so that it will always maintain the maximum width no matter what width you enter into the container/header/menu width rule. Add this to the bottom of your CSS and see what you think and then if you want to continue, we can get the header stuff straightened out for you.

#container, #header, #menu {
width: 1000px;
}

.hentry .main {
width: 100%;
}

Do you mean that changing the post column width will affect the header size? Yes I only really want to edit the column width of my posts, but if that means changing the header as well then I'll at least give it a try and see how I like it.

.hentry .main {
line-height: 175%;
}

This can be a little tricky as the way the theme is designed, the header image will also have to get wider. Do you want to do that as well?

#site-description {
    display: none;
}

Consider keeping a liiitle more whitespace then you might normally (just an idea/opinion on that, but give it a try). :)

Thanks so much for the help, and the welcome. Glad you like the blog design, it's a work in progress!

Would you perhaps know a way to get rid of that little shaded area beneath the title where my tagline would go? Not massively important, but if it was simple..

#header {
    min-height:100px;
    height: 100px;
}

Using my theme's original stylesheet as a reference, I've added this code to my stylesheet editor:

#header {
min-height:180px;
padding-top:40px;
padding-bottom:5px;
width:800px;
}

If I increase the 'padding-bottom' value it increases the space in question with no problem. But if I enter a small value - like 5px as above - it won't go any smaller than how it looks at the moment. Is it something to do with the space for the tagline, which I don't need?

Thanks for any help, I'm brand new to all of this!