I have recently purchased an Apple iBook G4. I have advanced chronic
progressive MS and am looking for the best way to move a mouse across
the screen by head movement, using a camcorder mounted on top of the monitor.
What is the best sytem that can be used for this? (including the
software required)

Hello Hamish246,

The options appear pretty limited and expensive - so I'll post this as
a comment instead of an answer.

Check out 
  http://www.gstsdesigns.com/Mice/alterative.htm
which shows several alternative mice - most are suitable for use with
head motion. The prices astonish me - as much or more than your new
laptop.

Hope this helps.

  --Maniac