So here's what I found:
First I built another machine with a fresh 11.4 install (which I was going to do anyway which is what precipitated this upgrading to 11.4 on this machine in the first place)
I did a diff between /etc on both machines
Not much was that different, but I did see that some entries regarding network in init.d that were different. I expected possibly differences in the config files, not so much in init.d.
But anyway, using YaST/Services in expert mode, I found the following diffs:
on the broken machine but not on the "fresh" machine:
avahi_dnsconfd was enabled
network and network-remotefs were both set for runlevel 2 in addition to 3 & 5 (only 3 & 5 on the working machine)

I disabled avahi-dnsconfd; and turned off network and network-remotefs at runlevel 2 (leaving 3 & 5 only).
Now the network comes up instantly as expected when using 'ifup'.
I don't know why this caused a problem (I never changed these directly myself before) but it did seem to fix it.

However, I still have the problem with 'ifup' that VNC drops the connection after logging in (VNC works fine with NetManger) - so I still have some sleuthing to do on that front. But otherwise I am back to a (mostly) usable system. The crucial services are now functioning again and I don't use VNC much anyway, so I'm basically back in business.

Thanks All for your help

On to fixing VNC...
Wow, what a good job of finding the difference there. The avahi_dnsconfd service is disabled on my two computers and I assume this may be associated with using your computer as a DNS server perhaps? In any event, good luck with locating your final problem. By the way, this procedure is still a good one to follow for updating existing installed software:

YaST / Software / Software Management / Package (menu option top left) / All Packages / Update if newer version available AND select Accept in the bottom right corner.

Thank You,