Rob, thanks for your technical response to my concerns. I still have a problem though, in that the rules say "You must avoid flying it within 150 metres of a congested area". I take that to be a horizontal distance. To me, the start of the walk was "a congested area", so as far as I can see, the drone should have been at least 150 metres from the edge of the crowd. Flying above the edge of the crowd doesn't seem to comply with the rules. I noticed on the day that you hovered the drone over the pathway that all walkers took from the start area; I even videod you landing it (at 16:00 into my video).
You say "... with participants being considered to be in the control of the organisers" but the rules refer to "people being in the control of the pilot", so they weren't under your control.
The fact that Richard Brown announced the presence of the drone has no bearing on the discussion. Neither does the fact that you've had positive responses to your video; indeed it does make an interesting video. The discussion here is about safety and compliance with the current rules and as far as I can see, the rules weren't followed.
Anybody interested in seeing a drone crash, should refer to the link that I posted earlier (although that one is larger than yours).
Here is a link to an Amateur Photographer discussion of rules for drones
http://www.amateurphotographer.co.uk/latest/photo-news/photography-and-drones-the-rules-4805