network install fails to access mirror

[Dave Howorth]

I'm trying to install karmic over the internet from a mirror (following
https://help.ubuntu.com/community/Installation/NetbootInstallFromInternet).

I've booted into the installer and it says it has used DHCP
successfully. It's now got to the stage of trying to contact a mirror
and has offered me a choice of archive.ubuntu.com and
gb.archive.ubuntu.com. It fails to contact either of those.

I can ping gb.archive.ubuntu.com from another machine on our net so I
know it's up and that there's a route. I can't ping the target machine
(Destination Host Unreachable) but I don't know whether I should be able to?

Is there some way to check the machine's network status or to see
exactly what commands it is using to try to contact the mirrrors? e.g.
Can I bring up a bash shell somehow (I tried CTRL/ALT/F2 with no response).

Thanks, Dave