Irvine is a very steep natural bowl and B stage was about half way up the bowl (it appeared to be the edge of section 4 as Dianne noted), and there is a grass verge that runs outside the lower walkway along the Orchestra 1 seats. I also think that's where he was running from.
It was cute, too, that when the band got to C stage, Chris commented that it was quite a hike to get up there.
Lovely show, altogether.