Anyone understand network proxies !

In my continuing struggle to get my newly "acquired" G5 Powermac
(10.4.11) up and running on my work's Network, I am running into a
problem connecting to Software update.

The machine is on the Internet and both Firefox (easy) and Safari
(struggle) are working fine - the latter wanted me to input several
usernames and passwords that I extratyed via Keychain on my old Mac.
Including one for isaproxy-ukbl.proxy-uk.xxxxxx:8080. Now I
absolutely know the username and password for that as evidenced by it
working fine on the browsers. However, when I go for a Software
Update it is requesting the u'name/p'word for the same Proxy Server
Authentication - but it just doesn't accept them ! It just cycles
back to ask again

is there something different about how SU connects ? Any ideas