Found a link to a Danish tv programme 'Technomania' from 1996 which contains a short clip (approximately 1 minute) from Underworld's gig at the Roskilde Festival 1994, where Björk joined them on stage, performing Human Behaviour (starts at about 17m into the programme). This is a 700 Mb file (split in two), meaning not everyone would consider it worth downloading
There are a few other interview excerpts with Karl and Rick elsewhere.
There's also a clip from the Orb's performance at Copenhagen Water Festival (which made it to the Live 93 album).
It's vhs quality and the sound a bit out of sync.
Link:
http://www.housemusik.dk/forum/showthread.php?t=12571
Go to the last message (#22) and download part 1 + 2
EDIT: The part with the short clip can also be found here
(at 7m40s) as well as the other parts..