Over the years Donovan has mentioned various people as playing on this track. It is now understood to be as follows according to John Paul Jones (who did play bass and book the session players) as:
Donovan on acoustic guitar, tambura and vocals
Alan Parker on Electric Guitar
John Paul Jones on bass, also the musical director
Clem Cattini on drums
John Bonham, Allan Holdsworth and Jimmy Page were not involved with this song; Jeff Beck, according to Page, did play on one version which was wiped. Jimmy Page said, "I know it's rumored that I played on that, but I didn't... I heard about this story actually when I was in USA, it was about the time we were talking about the deal with Led Zeppelin. We were at Miami with Jerry Wexler, and I heard about the story by there and then, across from England, and on the shores over here. And what the story was - and it's very true - that they had Jeff Beck go in, and Jeff Beck played on it, and the producer decided to wipe the track. And Donovan had asked for me to do it, but of course I wasn't there. And they had a guitarist, he basically filled, you know. He went into the session - and I wouldn't say filled my shoes - but he went in the door, and his name was Alan Parker. I mean, none of you even know of him. It's not the film producer. But anyway, he's the guy who played the guitar solo, so you know, as you say, some people might have thought Beck did it, or me, but it was neither of us. But I think it was tragic that Beck got wiped off. That was absolutely crazy. They just decided that they didn't like what he did. And I mean, perish the thought, you know."
Andrew - London, England for above 2 - source: liner notes from the 2005 reissue of the album