OK - Damn you Internet Explorer!!
After many, many hours I have what I need up and running. In case it's any help to anyone else I'll stick a quick summary below but if I could just check one more thing I'd appreciate it.
Will it cause any harm to have empty divs in the page code with no declaration in the main style sheet?
In order to maintain the curvy corner effect on that page but without the overflow from the expanding iframe (caused by the absolute positioning from the curvycorners.js) when the driving directions are loaded I've resorted to a conditional style-sheet for <IE9 which will hopefully recognise the border-radius declaration - I'll need to set up another virtual machine to test that.
In the page html I've added divs before and after the #wrapper and before and after the #main content block. These have png images with curved corners set as background, same with the footer.
These divs only have declarations in the false_cc.css file which will only be called by <IE9. This also adjusts the margins etc on various elements to make it all fit. I'm hoping these are simply ignored by any other browsers without any adverse effect.
I don't know if this is a monstrous hack or a respectable workaround but it seems to work and shouldn't cause issues with other browsers.
Thanks again to everyone above, I'd still be beating my head off a rock without that pointer about the positioning.