04 Xsara Picasso....DIM DASH

[kenno]

Hi All.....
My 04 XSARA PICASSO LX, after 9 trouble-free years has developed it's first fault.
The dash, though fully functional with all info displayed, and all bulbs OK, is DIM.
It used to be bright at start-up, and would dim when side / headlights were switched on.
It now seems to be 'STUCK' in night time dim mode. (Dim at start up, and no change when headlights switched on.)
The 2 button brightness adjuster operates as it should when lights are on, and is currently set at MAX brightness.
I've taken it to the local Citroen dealer, who insists that it is all OK, normal for an 04 model.
I might be getting old and losing the plot, but I just can't accept that.
Firstly, will somebody please confirm that the default condition is a BRIGHT dash for daytime, and that it DOES dim when lights are put on for night driving.
Secondly, if anyone has come across this fault, and could point me in the right direction, I'd be most grateful.
(Tried downloading multiplexing diagram, but it might as well be the Dead Sea Scrolls as far as I'm concerned.)