You probably know that the content comes from a web map. The Settings page of the web application has a data source URL which shows the global ID of the web map. The web map provides the data for the web application. See the pasted in graphic below.
Go have a look at the web map that underlies your web application. I believe the content in the web map can be embedded or it can be from an ArcGIS Online item. Hopefully it is the latter since it can be easier to work with.
Please feel free to respond if you have more questions of if this isn't clear.
I'm afraid that doesn't help me.
When i view the Settings > Web Mapping Application > Data Source URL - this gives me the appid of the StoryMap:
I'm not sure how i can use this information to identify which of the 50 or so files on my ArcGIS Online account are linked to this application?