Thanks Falko, I've made it through. :-)
The particular problem with debootstrap was that the instruction:-
Nope. That wasn't it.
The problem seems to be that I cannot get a full package download onto the mounted ext3 filesystem at...
This http://blog.invisible.ch/2006/03/09/installing-xen-30-on-debian-sarge/ helped me.
For the record, bypassing apt-cacher and squid may have been the cure.
I have created a debian minimal netinstall box and have been working my way through the recent Xen 3.0 HowTo. My only variation has been to use...
Separate names with a comma.