After 3 hours I've basically given up.
Initially I did not extract Rockbox utility (Ver 1.3.0) onto the Clip itself (doh) and attempted to run it from my PC.
Obviously this didn't work so I then tried to install it manually. I followed the instructions, safely ejected the device and the message on the device screen stated it was upgrading the firmware. When I turned the device on, the Rockbox logo appeared briefly with a "file not found" error, and then "usb mode" (or something to that effect).
I then booted with the standard Sansa firmware and formatted the device.
After this I extracted the Rockbox util and the firmware file to the device itself, and executed the util from the device.
This time, nothing, the original Sansa firmware boots and everything loads as normal.
I'd really like to use Rockbox. Please help!