Ah I wasn't at knebworth, I'm only 20 so will never get to see the band live, which sucks but oh well.
In '77 the double neck was used for Sick Again as it followed TSRTS but at Knebworth Celebration Day directly followed TSRTS. Here you go, from the 4th/8/79:
http://www.youtube.com/watch?v=EXTFwfGE0Ak