_Think_ that's done it. I'm still piecing together the workflow I am supposed to follow with git / github. In case anything looks screwy here's what I did:

git add / git commit --amend my existing commit in my local repository
git push -f    (it wasn't happy to push without (-f) forcing it due to the --amend altering the existing commit, I suppose)
This pull request appears to have picked up my changes.