I thought I was sorted, but today on further testing I have realised that I was hopelessly over-optimistic.
I have just twigged that a 'ppm' repository and a 'CPAN' repository seem to be different, even though you can find modules from CPAN in the ActiveState 'ppm' repository.
My original problem was that I couldn't install some modules downloaded from CPAN and also couldn't install some modules interactively using 'MCPAN'. However I could install using 'ppm'.
I hadn't realised that 'ppm' uses the ActiveState repository and not the CPAN mirrors.
So far I haven't found the descrition on how to download the ActiveState 'ppm' repository for local use or how to convert a CPAN repository (with the 'author' tree and '.tar.gz.') into a 'ppm' repository by adding 'ppd' files.
I may have to backtrack and fix my CPAN problems after all.