Unheard song revealed ahead of September 19 release of '20,000 Days On Earth' film
Bad Seeds frontman will perform with Warren Ellis and Barry Adamson at London event
The Bad Seeds man will play live after select showings of '20,000 Days On Earth'
The promo features a new live version of the track
Band unveiled the song on the opening night of their just-finished UK tour
'Live from KCRW' is released on December 2, 2014
The gig will feature in Cave's forthcoming film about his 20,000th day on earth
The band will play Cave's adopted hometown on October 24
'They don't want their children around words like snatch,' says Bad Seeds singer