system wide http proxy settings

[grep at westnet.com.au]

When installing ubuntu, there is an opportunity to set a system wide
web proxy. I'm trying to find where this info is stored (or a command
for changing/setting the proxy details). Access to the big wide web
from the pc currently works, but I need to change the proxy
connection details. It's a headless server (no X apps installed:
minimal server system only) so access is only via ssh.

I am aware of the http_proxy variable, but this doesn't seem to be
the mechanism being used. In addition, when I have tried using this
(on another system on the same network) there are problems with the
proxy requiring authentication.  Setting http_proxy to
"user:passwd at address:port" doesn't seem to work.

Anyone able to point me in the right direction?

Thanks,
TB