Sorry, Floppy, looks as if the +1 goes to me instead ;-) This assumes, of course, that Xorg continues to run correctly on this machine (and is, indeed, operating with acceleration).
I had another look around the internet at general Xorg ModeLines, and realised that I'd got the syntax in all of the ModeLines wrong.
I changed this:
Modeline "1280x1024"x60.0 108.00 1280 1328 1440 1688 1024 1025 1028 1066 +hsync +vsync (64.0 kHz)
to this:
Modeline "1280x1024x60.0" 108.00 1280 1328 1440 1688 1024 1025 1028 1066 +hsync +vsync
The reason Xorg couldn't parse the line, or find the dotclock, was that x60.0 was the first item after the double quotes, which is where the dotclock frequency should be. In addition to this change, I removed (64.0 kHz) from the end of this line and the other ModeLines, though I doubt it was a problem. As I'd changed the start of the ModeLines by moving the double quotes, I had to change the PreferredMode string accordingly, and the Modes string in the Screen subsections.
I'm writing this with a 1280x1024x60 screen resolution, and a British keyboard layout, so it looks as if the xorg.conf has worked fine, but only time will tell!
Many thanks for your help, Floppy, and thanks too for the help of everyone else who contributed to this thread. Apologies I'm such a slow learner!
Cheers, Jon.