By now I'm not even faintly surprised at what this program does. I have a texture selected in the color texture node, it's connected to the diffuse and highlight sockets and it shows up nicely in the preview window, wth Jessi on the flat roof looking at the sunset. Do any render and J and the house are there, with lovely long shadows, but the ground has vanished. 

I'd show you the screen shots but this thread won't allow any insertion of images. It deleted my text when I clicked on the icon last try.

The manual ,as usual, is deafeningly silent on this topic : anyone know what the (probably simple but wonderfully well hidden) answer is??

Go into the material room and select the Ground.  Down at the bottom of the nodes make sure Shadow Catch Only is unchecked as this will keep the ground from showing.

Sopt on, VL, many thanks. 

I knew somebody would know it. Membership of a forum like this is worth 10 manuals

You have to host your image somewhere else to post on these forums (from what I can tell).
I use imageshack.