Can I add a "sub-container" around the lefthand column and righthand columns but NOT around the masthead?
I want my masthead (and the container behind the masthead) to be transparent so that the words of the masthead appear directly over the background picture of the body section.
However, I want a coloured backgound on the "subcontainer" around the left and right hand columns so that this background extends down to the footer on both the left AND right hand columns even if one column is longer than the other. Setting the background colour of the left and right hand columns gives me one column visibly shorter than the other.

Probably!

For anyone interested, I think I've sort of!! fixed this by adding a subcontainer into my master file and closing it after the footer (closing it before caused problems in Firefox):

<!-- Begin SubContainer -->
<div id="subcontainer">
<!-- Begin Left Column -->
<div id="column_l">
<!-- #BeginEditable "content" -->
<h3>Headline 2</h3>
<p>insert content here</p>
<!-- #EndEditable --></div>
<!-- End Left Column -->
<!-- Begin Right Column -->
<div id="column_r">
<!-- #BeginEditable "right hand col" -->
<h3>Heading</h3>
<p>insert content here</p>
<!-- #EndEditable --></div>
<!-- End Right Column -->
<!-- Begin Footer -->
<div id="footer">
footer code inserted here
</div>
<!-- End Footer -->
</div>
<!-- End SubContainer -->

Not ideal but I can work around this and sort the footer out.

It's very easy to do it - the best way to work out how your divs are going to work is by drawing them onto paper first - It's a method I have always used