I have managed to work out what the issue is.
When a member uploads a video using the 'Insert Video' method then the video retains .mp4 in the link. However when a member uploads as an attachment the video link is referenced which cannot be viewed by browsers using the iOS (operating systems) on Apple products
As a test, the following are the same video but uploaded using the two different methods:
The issue is when a member uses the 'Attach file' button to upload the video but then doesn't insert it into the post once it is uploaded.
What seems to happen is if it is not inserted the video is not moved into the relevant 'data' folder but remains sat in limbo as an attachment with a file reference that iOS cannot read.
Here is the before and after link using the 'Attach file' button on our forum, and then using the option to insert video: