Thanks for the feedback Angela 
It does make sense that my arm movement during the start phase could be too long, and would then be getting out of sync with my foot strike rate and causing the stumble. I think arm drills are definitely required though, as I don’t tend to do many arm specific drills.
I just watched the CF video, very informative! And what’s being said does make sense, although when I then see a video like the one below it seems to contradict what they are doing? (arms not at 90 degrees and swinging back much further)
[video=youtube_share;4QrlPmK4B94]http://youtu.be/4QrlPmK4B94[/video]