One further thought in relation to this:

I've also created one by creating a /somepath/cpan/authors/id/A/AL/ALL directory, and copying all the files into there, but that mucks up dependancies.

I just went a little further on this - if you recreate the 02packages.details.txt.gz file with the correct path in the 3rd field (i.e, change to A/AL/ALL/<module.tar.gz>), and then wipe the Metadata in ~/.cpan, it figures out the dependancies properly and grabs them from the local cache - that would alleviate having to create the correct directory structure, and it also shows that 02packages.details.txt.gz is the key file to ensure is consistent.