A bit of clarification re the OGWT Live Box set seems only two CDs from the original TV show and the other a compilation:

"CD1 and CD2 feature tracks from the original classic show including Van Morrison, Lynyrd Skynyrd, Elton John and the mighty Tom Petty and the Heartbreakers. All original and all live - CD3 features tracks from the Old Grey Whistle Test's 40th anniversary BBC Radio 2 show as produced and presented by Bob Harris himself in 2011. All the artists featured on CD3 performed on the original show at some point"