Once I'd convinced the Windows 7 machine to find the relevant folders and copy the images I wanted to keep over there was then the problem of deleting what was still there. But even after having visited the Photos app and deleting everything, the app itself indicated that it was still holding a mass of data of some sort I couldn't see.
To Google and once of the most bonkers technological quirks since Y2K and a fix found on Apple's discussion boards from a user called scabthepoet which I'm going to reproduce below so that it's in more than one place. I can't find more identity details for them so if it's you and you stumble here and you'd like me to take this down or provide better credit, let me know, but know you're an utter, utter genius:
- Go to Settings
- Date & Time
- Untoggle "Set Automatically"
- Manually change the date back. For example, if today is March 15, 2015, choose August 1, 2014. (You can change it back once we're done)
- Close out of that
- Open "Photos"
- Select "Albums"
- If, like me, you had already cleared out everything from the Camera Roll and "Recently Deleted" folder, you'll smile to see that your "Recently Deleted" folder now has thousands of images back. Those are your phantom photos
- Open it, "Select" and start deleting
- Now, go back into Settings - General - Usage - Storage - Manage Storage - and you'll notice your Photo & Camera is empty if you deleted everything
Just as an aside, the trick seems to be to take the date back to way before you bought your product - I went to 2012. Later than that doesn't seem to work as well. Let me know how you get on.