Split a commit into smaller commits with Git?

Question

I copied a file and made a bunch of changes to it at the same time (i.e. in one commit). I then carried on making changes in subsequent commits. I now want to split that commit into two - one that just copies the file to its new location and one that modifies that file. How would I do that? If it was the most recent commit it would be easy, but this is way back in history now.