OK, I've fixed my problem - the absence of a number of colour shades, mainly
due to the loss of RED. I found that I could print the full range of colours
OK, even though they did not show up on the screen, suggesting a monitor
problem. Somesone suggested looking at the monitor connection, which I did.
I found there was a poor connection/bent pin on the connection from the
monitor to the computer. Once I fixed this, the rainbow of colours returned
to my screen.
Many thanks for everyone's help, David