I was a bit confused as to why I needed to add a hidden field as have not had to do that before... so started it from scratch and is working fine now.
With the same site that this issue happened on there is something else happening that I dont understand and will struggle to explain.
Before I started from scratch the image files were being uploaded to uploads/sponsors/
For some reason, a folder (drivers.php) kept being created and every time I deleted it, it returned. I have, since starting from scratch, changed the upload folder to images/sponsors/ and the exact same thing is happening in this new folder and not the original.
Have attached a screenshot from Filezilla showing the folder tree of this folder that keeps being added and re-added.