So, over 20 emails later and much reassurance that the YouTube technical team are 'looking into it', I've realised that nothing's ever going to change with the video, so I've made the rather heart breaking (in view of all the positive comments and interaction it'd received) decision to de-list that video and start again...
This is the EXACT same video file, the only difference here is that I uploaded this to my Google Drive first (for the YouTube technical team to so-say download and look at) before I decided to try exporting it into YouTube to see if the correct resolutions would show up this way.
I've got a bit of plea here: If you were one of the people who were kind enough to leave a positive comment on it originally, please can you go back and comment again, give it a thumbs up, share it, anything really that'll help it get noticed by the YouTube algorithm to try and help it get noticed, so it doesn't languish in no views hell. Thanks in advance if you do - you're ace!
Correct resolution version:
https://youtu.be/WwK2HlKXHJI