As far as I know, Ralph McTell and Al Stewart devised the tune together, or at least both agreed to use it, and wrote different words to it. Al recorded 'Samuel Oh How You've Changed' before 'Streets' was recorded, though not long before. I played support for Al sometime about 1969, but I can't remember if Ralph's version was out then. As for Pachelbel, look in a dictionary of musical themes and see how many classical themes match each other - there's yards of them.
KYBTTS