I had a look at /etc/X11/xorg.conf
And found an entry in the monitors area that included the string Syncmaster - referring to the LCD connected to the machine that I installed Debian on. I tried replacing this with Configured Monitor, Default Monitor, Plug and Play Monitor which were used in examples of xorg.conf I found searching the web. None of them worked - still got the same error.
So I'm guessing that another file might be involved which we don't know about.
It's probably easier for me to just reinstall with the CRT monitor attached - as this monitor will be the one that is going to be used.
But I will say this again, I think this is a concern if people wish to move machines around their premises and use different display hardware from that used in the install.
HOWEVER, I have found a very promising book - "X Power Tools" by Chris Tyler published by O'Reilly in 2007 (great! Fairly recent!) which devotes TWO whole chapters to configuring the X Window System! This looks very encouraging.
There is a, perhaps very telling, section called "Places Your Configuration Could Hide" which might help me solve my problem.
I'm loathe to buy yet another book, having spent a considerable amount already on other books. Particularly if I know I can get around the problem by just doing a reinstall (but I don't know yet!) My curiousity may overwhelm and tempt me to buy the PDF download though. I'm mentioning the book in case anyone else has had the issues, or similar, as I have. Good old O'Reilly! And good old howtoforge.com