I was there ☺️
Slade were brought in to replace Ozzy's Blizzard Of Oz, and apparently were quite hesitant about doing so.
You are correct about their prowess as a live band - and Noddy had us eating out of his hand before long. He jokingly said it wasn't a 'request show' as so many people were shouting for them to play Merry Xmas Everybody, and followed that up, in his thick Brummie accent with, "Go on then - you lot f***** sing it!". And we did! (they even kept the crowd sing-a-long on the Slade Alive IV/Live at Reading 80 album).
Great line-up that year for us (New Wave Of British) Heavy Metal fans. Iron Maiden, Def Leppard, UFO and a load of others, plus established rockers like Rory Gallagher, Ian Gillan, Gary Moore and (the simply wonderful) Whitesnake.
And incidentally, I was a big Slade fan as a kid - Gudbuy 'T Jane was the first record I ever bought ?