Will, you are the second person I have heard today reminisce about a John Mayall gig (the other was at The Boat Club, Nottingham) Actually the late, great Dick Heckstall - Smith was also a founder member of Colosseum, what a band, and Jon Hiseman actually wanted him as co-leader of the outfit.
Diane, Klooks Kleek was the first London appearance of Cream according to Chris Welch's book "Cream". Prior to that the had played The Twisted Wheel, Manchester and The National Jazz and Blues Festival, Windsor