Sproggy - thank you for a wealth of info I didn't know. The crucial bit for me is your last para -"When you upgrade you have to upgrade everything ... you also have to be aware that Xfce does not like cross polination of the desktops ... and Xfce 4.8 is more stringent in this than any before it ..."
Firstly I did upgrade everything. And as a matter of interest, I was using kernel 188.8.131.52. But I had no idea of the situation re cross-pollination. I have no 'must have' KDE apps in the usual sense of the word. But being, amongst other things, a photographer, I need Digikam. OK, I could use Photoshop I suppose, in Windows in a virtual box, but I know Digikam now inside out and everything I do with photographs I do on that. So I think that's going to keep me from Xfce - certainly at least on my main machine. But as I've now successfully installed it on a machine I use for testing I shall be using it often and will be in a position to test it out thoroughly and without pulling in any KDE stuff. And I'll upgrade and see how that goes!!
Thank you for your very full post. I think on that basis I should mark this as solved. Much appreciated.