Some photos aren't displayed when iPhone connected to PC
After much frustration, Googling and trial-and-error, I came to the conclusion that the photos displayed on my phone weren't actually on my phone, but were instead "optimised versions" of photos stored in iCloud and therefore inaccessible through the file structure when connected to my PC.

(It should be noted that the photos did not appear pixelated or blurry in any way in the Photos app on my phone, but the following solution worked which indicates to me that these were "optimised versions"):


Navigate to Settings > iCloud > Photos on the iPhone
If "Optimise iPhone Storage" is selected, you have solved your problem:  this means that some photos appearing on your phone are in fact stored in iCloud
Select the option "Download and Keep Originals" - this will ensure all photos are actually stored on your phone, meaning they will appear when connected to a PC via USB.


Note: You may find that selecting this option results in a message that your phone lacks the necessary capacity to download originals of all your photos. I had to work in stages, allowing my phone to download the originals which I then copied to my PC and deleted from my phone, creating more space for more originals to be downloaded.