Thank you for your interst in this topic, Frank and Leonie. You, and possibly other readers, may be interested to know that I have managed to do what I want using the channel invert filter in Final Cut Express. It involves quite a lot of steps, including taking a screen shot which is of a low quality, but it has given me a positive from my negatives.
The question always is, what's it to be used for?, and in my casse I just need a few old family photos to add to the story I'm writng of my childhood. Let's face it, the originals are not that great, and a fairly low quality final product is adequate to include as small photos in a Pages document.
For higher quality needs, and inverting some of my colour negatives, I will likely have to go to Aperture, along with its other many benefits.
Thanks again, Keith