Well it took a couple of attempts but l got it loaded in the end.
For those that follow:
The set-up is a Toshiba Portage M200 tablet with 1.6 Pentium M and 2GB of RAM.
I've swapped out the original drive with a CompactFlash drive (16GB, X133) using a Startech IDE converter (I've not been able to get the second drive working reliably yet, likely a bios issue): 2.5in IDE to Dual Compact Flash SSD Adapter Card - StarTech.com
I set the drive up using the second set of instructions here: USB Windows 7 Installation Key Drive - Create
This recognised the drive as the insilation DVD on boot and proceeded with the installation normally. When asked for the instal location l selected the same drive. This seems to have worked Ok. Once the installation was complete I deleted out the old DVD files. All that's left is to sort out the boot sequence as it still lists the DVD as a boot option.
From a cold start to the login screen: 25sec
Login to desktop: 15 sec
Copy: 112 MB/S
Create: 73 MB/S
Read: 302 MB/S