OK, after consultation with the core dev team. I've been advised that a slightly better option (does not require another user to approve my changes to their PR before core committer is able to see them) is to create a new Pull Request based upon the previous Pull Request by user_x.
- Fork the core repository and clone it locally
- Add the user_x fork of core as an additional remote to my own local repo.
- Pull the PR branch from user_x fork of core to my own local repo
- Make my changes locally to the branch
- Push the PR branch back to my own fork of core on Github
- Generate a new Pull Request in my Github fork of core against actual core 1.x
This blog post was helpful - https://tighten.co/blog/adding-commits-to-a-pull-request