Hey Paul,
Do you want them to remove the video?
If so, send them a registered letter requesting them to do so.
You could soften the language and open up negotiations with them for continuing to use the video.
If that fails, ask Youtube to take it down based on copyright violation. This will be your point of no return.
Don't forget to name and shame them.
All the Best
Mads
@madsvid, London, UK
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http://mads-thinkingoutloud.blogspot.co.uk