There are a number of lyrics sites (not the most reliable source, I know) that list Kit Hain as the author and both Mick and John as "publishing participants".
I'm not entirely sure of what a "publishing participant" is but the book "The Plain & Simple Guide to Music Publishing" seems to suggest it is someone who gets publishing royalties for the song, possibly without being an actual author:
https://books.google.com.au/books?id...yright&f=false
Perhaps that was the price of getting the song onto the album?