I ran into exactly the same problem with another song in the book. One person claimed to have written it all, the other to have written most of it, and neither had ever got any of the considerable royalties it earned. They both thought the singer who made it famous had got all the cash. In fact it stuck to the fingers of the publisher. One of the co-writers fell out badly with me about what I wrote in the book, but relented when I managed to get both of them some hundreds of pounds of the earnings, and sorted out a secure base for future earnings. Mind you, neither of them gave me a penny or a bottle of booze in return.
As regards Norman, I dedicated the whole book to him, acknowledged his involvement in the song, reported what Archie said, and printed only what PRS / MCPS said was the correct credit. Failing memories might be involved, but the PRS / MCPS credit has been that way for many years, since about the second time the song was recorded.