How Ozzy Osbourne and Black Sabbath found their sound - and invented heavy metal

Mark SavageMusic CorrespondentGetty ImagesBlack Sabbath in 1970 (L-R): Geezer Butler, Tony Iommi, Bill Ward and Ozzy OsbourneIf you saw Black Sabbath's first ever gig, you wouldn't have recognised greatness.Back in 1968, they had the decidedly less sinister name of The Polka Tulk Blues Band, and came complete with a saxophonist and bottleneck guitar player.A year later, they'd slimmed down, found a new name and…