A few minutes later...
Pasting the URL from the concert page didn't work because it was asking for some password which wasn't the one that identifies me into the video site, nor anything OSX related. On two trials using slightly different URLs it just was getting the trailer of the concert, but not the concert itself. However if I used the Chrome add-on of Video Download-Helper to choose one of the videos from the list (it's a PITA because long concerts come in separate videos for each movement as far as I could see) to grab the URL, then pasting it into AllmyTube voilá it got a nice 14 minute video at pretty good resolution in mp4 so no further conversion needed. It's a bit of work but I think less than the VDH and then converting using ffmpeg. Probably worth paying the fee. Thanks for the heads up