can't change desktop wallpaper

[Roger Searle]

One of the simplest desktop configurations - changing background picture 
- is defying any attempts to change.  Makes no difference if I try to 
apply to all desktops or a specific one.  Nor can I configure colours, 
or specify "no picture".  I always get a solid deep blue colour displayed. 

Anyone struck this before or know what might be stopping it being 
configured?  kubuntu gutsy.

Cheers,
Roger