Thanks Torsten, that's what MCP told me and it works with items already in the Desktop folder.
However, somehow in my experimentation, I seem to have disabled the "Add item to desktop" facility. I find the item in the menu tree, right click on it and then left click on "Add item to desktop" in the dropdown menu but nothing happens now. It did work earlier, and the "Add item to panel" button does still work.
It was possible to move the icon from the panel to the desktop, and it worked, but did not appear in the Desktop folder. I then examined the properties of the icon and found it pointed to /usr/share/applications/KDE. So I copied Konquerorsu.desktop from this folder into my own Desktop folder and, hey presto, it worked from there too. So it's just the copying routine from the context menu that is disabled.
No doubt it will return when I do a full re-installation when 2010 is available. At the moment I'm just testing, but it would be interesting to discover if I can fix this.