Ok just realised UnityEditor only works for things… in the editor (i.e the heirarchy window not the game tab) 
Additional alternatives (and some interesting links) here for now - but i’m determined to solve this!
- https://stackoverflow.com/questions/65792831/how-to-use-c-sharp-to-open-windows-explorer-in-select-open-file-mode
- https://www.google.com/search?client=firefox-b-d&q=Open+File+Dialog+++Unity
- Unity - Scripting API: EditorUtility.OpenFilePanel (i think this won’t work in a build because it’s editor utility) - OpenFileDialog in Runtime