It now appears that I had got most of it right. This might be a red herring, but so long as I don't try to write to the last character of the 4th line of the display it all now works. If I fill the 4th line then the whole display blanks.
If I can definitely nail this down where should I file a bug report.
So I've revised my screen layouts to avoid filling the last line. 19 characters O.K, 20 characters = pear shaped.
I've got the X,Y,Z positions coming up on the display and they track the values on the main axis display.
Onwards and upwards now to explore the mystery that is matrix_kb.
Thanks for all the help everybody, and I'm sure I'll be back again with some more questions.
Roger
