image 404 error (via JSON:API)
I was able to delete the files via JSON:API (thank god that exists) using Postman.
The FIDs were: 1,2,8,9 - indicating that the files were very early on in the development of the site. I think they may have belonged to article content types that were recently removed (and therefor orphaned) - however I was still able to build the gatsby sites after this.
However, I think it was straight after I deleted the .cache folder from the Gatsby build that I had this issue, so it's possible that I had the images cached, they were then orphaned (during deletion of the 'article' content type.). Although I would have expected that Drupal would have done appropriate deletion of files and records when removing a content type.