Well, yesterday was not so smooth as I had hoped - though the LMCE installation was fine, the VM wouldnt actually boot up. After much trying, I gave up on that and started again but did a LMCE dedicated install. This now works but took another day to get there.
Today, Ive been fighting with gently poking virtualbox's network settings - I cant find settings that let me use my network and do a network boot but I think I now know how to set each up. So, onto the network booting...
As has been said before, Cromwell wont boot from the included pxelinux.0 file. I tried the latest version of PXELinux - didnt help. I looked online for stuff about the error ("error not a valid image, unable to load file") and found somewhere that said etherboot does not like booting pxelinux (Im not sure if this is correct - I used etherboot to boot my desktop from my laptop LMCE installation and that worked fine - thoguh that could have been a later version of etherboot that is in cromwell) and suggested using mkelf-linux (from mknbi) to make an ELF for etherboot to run. I tried mkelf-linux with the append from pxelinux.cfg/default and the initrd & vmlinuz from default/ (made a 6mb file). This definitely worked better - lots of .'s printed onscreen (presumably, indicating it was downloading the file). Unfortunately, the .'s went off the bottom of the screen but I watched the network light on the XBox - solid/rapidly flashing for a while then stopped. Unfortunately, Cromwell didnt do anything after that 0 Im guessing there may have been an error message but because the .'s went off the screen, any error message was also off screen. However, when the first error ('unable to load file') was raised, Cromwell rebooted itself after a while. After the .'s it sat there for at least 30 minutes without doing anything (visible anyway).
I wasnt going to do anymore on it till tomorrow but writing this has made me want to try more - only useful idea is mkelf-linux without the initrd and hope it gets it through tftp. Ill see what happens...
Nick