G'day MiG and Vespine,
Many thanks for your debate as it has opened my eyes to the quality involved with camera lenses.
For my level of photography, this CA problem is nothing more than a minor issue to be dealt with at leisure by post processing (if I really feel like it). I tend to believe that it is the lens at fault as opposed to the camera body simply because I have not noticed the problem in images taken with the 18-55 or 28-80. When time permits I'll go through my film images and pick out any 75-300 shots and try to pick out the CA...although it could be hard in a standard 6x4 print.
Peter.