Hi Rich
I took a further look at the video. In it he refers to a program called Android File Transfer. When checking their website I see links to the program for use with Mac. On using the link it now appears to be Windows only. Random internet comments seem to confirm that this is the case currently.
I then resorted to using Macdroid and after a tortuous route through the file structure I found the cached mp4 files.
For anyone following the thread they were here:
storage / emulated / Android / data / dci.go.v5 / files / media caches
Thanks for taking a look at this for me. much appreciated.
Chris