I loved, loved, loved this show when I was a kid (I saw it as a syndicated show in the US around 1971, I am guessing). Yes, the soundtrack was the hook: I came for the (at the time) great theme song, but I stayed for the mystery of the UFOs, the spaceships and moon base, and Michael Billington (who, for some reason, I liked better than the star, maybe because he was younger and actually went out into the field).

Edit: Interestingly, Ed Bishop and Michael Billington both died within five days of each other in 2005, despite Billington being quite a bit younger.