Indents need to be controlled in the Define New MultiLevel List dialog.
Set your numbering up following the steps in How to create numbered headings or outline numbering in Ribbon Versions of Word by Shauna Kelly.
Then change things by changing which style is applied, not using the numbering buttons.

Thanks. I’ll give it a shot so.

@Charles Kenyon would you have a video by any chance of how to do this? I'm getting a bit bamboozled by these instructions.

Prephil, maybe you would find this video useful. When you go with this method, remember to start with the first instance that will be numbered and work front-to-back while adding these to an existing document. Once you have your numbering lists built, you can associate them with heading styles as well.

https://www.youtube.com/watch?v=He_ob8ydc9E