is my next guess.... Target Luna followed by 3 Pathfinder series.. Pathfinders in Space,Pathfinders to Mars and Pathfinders to Venus.
not easy to find...(assuming i got it right this time) .
To explain the clues:
Clue 1 - all four series aired in 1960-61 - they did
Clue 2 - the hero's pet mysteriously changes species (but not name) between the first and second series. - It was a hamster called Hamlet in the first series, which became a guinea pig (still called Hamlet!) in later series.
Clue 3 - It's live action with some models etc., not puppets. - model spaceships etc, can't remember what they did about the dinosaurs etc. I really must watch the DVDs, I've got the set somewhere.
Clue 4 - The odd series out was actually the first (sorry I didn't make that clearer), does not survive except as a script, and has a two-word title.
All of the others have a completely different cast from the first but playing the same roles, have three word titles with one word in common, and survived (via various foreign broadcasters, collectors, etc.) to ultimately make it to DVD.
Pathfinders in Space,Pathfinders to Mars
and Pathfinders to Venus
Clue 5 - One of the adult stars played his first role as a journalist and pilot in these series, later played a different journalist in another three more borderline SF serials, and a member of British intelligence in an espionage series, but is probably best remembered for a much later role as a robot on Doctor Who.
Gerald Flood -
played journalist / pilot Conway Henderson in the Pathfinders
Journalist Mark Bannerman in Plateau of Fear, City Beneath the Sea
and Secret Beneath the Sea
Peregrine Smith in The Ratcatchers
Kamelion the robot in Doctor Who (1983-84)
Over to you!