I want to dedicate this post to Bartek Bielawski who has already blogged the solution to this problem. I used the en-GB build of Windows 8 to build my desktop and a few other machines and have been failing to get the RSAT for Windows 8 RTM to install. Bartek found the issue to be caused by the fact that you can only install the RSAT tools on machines whereby the en-US language pack is installed.
His post with how he found the answer is at http://becomelotr.wordpress.com/2012/09/17/rsat-on-windows-8-works-but-fails/.
To resolve the problem, I have downloaded the Windows 8 Multi Language x64 DVD, and installed the en-US language pack from it, which now begs the question, what is the difference and benefit of running the localized build if you have to install the en-US pack anyway?