I believe that John Gielgud tipped Peter as a major talent back in the early days.
Dept S and Jason King were the end product of his early roles in similar shows, except that in the earlier appearances he played baddies - but charismatic baddies. He was in the Avengers twice including, I think, the 'infamous' Touch of Brimstone (in which Emma Peel appeared as the Queen of Sin). He was also in the Saint, and made a good Number Two in an episode of the Prisoner.
Don't know about the Bill Shatner album, Herlock, but I've heard Leonard Nimoy on LP, and it's a trip!
Afraid I know little about Mr. Wyngarde, but I've seen "Department S" discussed on a board for fans of espionage fiction and drama. It was apparently well-known in its day, though the still posted above is a bit alarming. Will need to check the "Doctor Who" wiki to see which episodes Wyngarde appeared in.
Shatner has done more than one album I believe. I know that he did one in 2011. There’s at least one on YouTube. Here’s a song from his 2011 album. I call it a ‘song’ but Shatner never actually sings.