I (eventually) figured it out. I did two additional things but I'm not sure which of those actions helped or if both of them helped.
Like I said before, I had gone into YaST > Software > Installation into Directory and installed about 1GB of KDE libraries and apps as the Standard System. Having done that I reasonably assumed that I had a complete KDE environment to go. But then I went into YasT>Software installation and found a whole load of KDE libraries that had'nt been installed. So I basically ticked everything that looked like it might remotely be connected with KDE in some way.
Also, after looking at yet more forums, I found a post that said you should log-off rather than reboot to get the session menu. Yeah, I know thats obvious to a lot of people but no-one had said it specifically, and being a newbie it hadn't occured to me.
Anyway, there it is. Live and learn.
Gilbert