Hello Mr. Rees
Mine is a Libreboot laptop. When I select boot from an isolinux image from a CD/ DVD, the CD is read, but after some time, the cursor drops back to the same menu line. When I checked the contents of this CD, what is mentioned in the below email were available...
Libreboot boots other CD with isolinux images

Is there a way in your machine to select whether to boot from hard disk or cd/usb etc.?  Something like F9, F12 after some bios splash comes up but before the isolinux kicks in.