This is a stubborn one! The only way I could get the slideshow_dissolve and slideshow_blinds to have a sure 960px width was to change the width:960px to min-width:960px. I thought that was it, but then some of the widths started going full-width. So I also added a max-width:960px as well.
The contentWrapper rules block now looks like this:
margin: 0 auto 0 auto; /* Sets the margin properties for an element using shorthand notation (top, right, bottom, left) */
text-align: left; /* Redefines the text alignment defined by the body element. */
This seems to (largely) do the trick - See Here - but there's still something going on with the footer in a couple of the designs. I'm sure I'm missing something here, but now have screen/css blindness!
Will have another look tomorrow.
Footer problem fixed.