Further to my previous posts "Boot problem with external drive" I have at last got it to start booting.
Wayne's link provided the solution. I included an initrd image with a built in delay, waiting for the drive to 'appear', and it has now started to boot.
However when I started to get some odd messages, during boot, I realised that the drive I'm trying to boot from was on my desktop, with one network address, and the laptop I'm now using to boot it is assigned a different network address causing the poor thing some confusion.
I shall try and boot it again, but not tonight, without the desktop connected to the network so that I can give the laptop the address normally assigned to the desktop. Then we shall see what happens.
Thanks to the people who provided help.
Barry Samuels http://www.beenthere-donethat.org.uk The Unofficial Guide to Great Britain