projectitis

I’ve just gone through this. Just in case anyone wants to do this in the future –

1. Make a child theme based on twenty ten
2. Add the following css to the child theme css
3. Copy header.php from twentyten to the child
4. Make one change in header.php to the header image as below (around line 105)

add to child theme style.css

*{
	box-sizing: border-box !important;
}
#access,
#access .menu-header,
div.menu,
#colophon,
#branding,
#main,
#wrapper {
	width:100% !important;
	max-width:940px;
}
#branding img{
	width:100% !important;
}
#site-title{
	width:70% !important;
}
#site-description{
	width:30% !important;
}
#content{
	margin-right: 30% !important;
}
#primary{
	width:25% !important;
}

@media (max-width:899px){
	#primary{
		float:none !important;
		width:100% !important;
		border-top: 1px solid #e7e7e7;
		margin-top:24px;
		padding-top:24px;
	}
	#primary .widget-container{
		max-width:480px;
	}
	#content{
		margin-right: 20px !important;
	}
}

Change this line in child theme header.php
<img src="<?php header_image(); ?>" width="<?php echo esc_attr( $header_image_width ); ?>" height="<?php echo esc_attr( $header_image_height ); ?>" alt="" />
to this
<img src="<?php header_image(); ?>" width="100%" alt="" />