I remember, back in 1999, the Italian TV channel TMC 2 (formerly VideoMusic) aired the Queen - Live At Wembley '86 concert from the official tape, albeit slightly butchered (not that the official tape wasn't already), but a week after that, the same channel also aired the Knebworth Park '86 concert, although I missed it.
This means, that that channel has to have at least 75% footage of the show. Now, the channel doesn't exist anymore, though its current owners, Telecom Italia Media, also own two other TV channels, La 7 (formerly TMC - TeleMonteCarlo), and MTV Italia, which is the Italian branch of MTV - Music Television, and is currently broadcasting on the exact same frequencies, on which before, there was TMC 2/VideoMusic.
So basically, the company has to still have the footage. And yes, it was indeed Knebworth Park '86, it was even advertised to be aired as the last concert Queen ever had with Freddie Mercury, if I re-call correctly.
But, if this is true, then this means, that Queen Productions must also have that footage (after all, TMC 2/VideoMusic had to have got the footage from somewhere), and if not, then VideoMusic must have had cameras present on that concert which recorded it all, and maybe even broadcast the show at some point in 1986, or maybe, 1987.
Anyway, this is certainly worth investigating, in my humble opinion. ;)