Re Lloyd and Hugill before 1961:

Hugill writes that in the mid '50s he "contacted the Folk Song Department of the B.B.C." and "recorded several of the rarer shanties for their Permanent Records Library." He "also became known at Cecil Sharp House, the headquarters of the English Folk Dance and Song Society, where I was asked to give talks on the subject of shantying and shanties and where I met many well-known collectors ofsea-songs and shanties, with whom I exchanged notes on the subject."

Hugill even refers to "an excellent recording of sixteen sea-songs (forebitters) and shanties sung in fine imitation of the true style, and in particular the Liverpool seaman's style." This must be the Topic LP "The Singing Sailor," issued 1956/57, featuring Lloyd, MacColl, and the actor Harry Corbett

http://www.informatik.uni-hamburg.de/~zierke/lloyd/records/thesingingsailor.html

"Stormalong John" appeared on the 1957 Topic sequel, "The Black Ball Line," with MacColl as shantyman.   

It thus seems probable after all that Lloyd and MacColl got "Stormalong John" directly from Hugill rather than the other way around.

MacColl's "hitches" in the shanty (probably never heard before on a commercial shanty recording) are more likely to be from Hugill's influence than to be a component of an elaborate counterfeit.

The simplest explanation seems to be that "Stormalong John" came to MacColl from Harding via Hugill.

I don't believe Hugill would ever have claimed he'd learned "Stormalong John" from Harding if in fact he'd only heard it on a "revival" record (or even on a B.B.C. recording) just a few years earlier.