These are the two books I can find written by Harry Hanson, both in the 70s. 'The Canal Boatman 1760-1914' is a detailed, heavily referenced work published in 1975, adapted from his MA thesis at the University of Manchester. It's a sociological work, really, and an interesting read. Inevitably a lot of the written record is about misdemeanors, but it certainly challenges some of Rolt's romanticisms of life on the water.
'Canal People' is a popularised version, in a very typical David & Charles style, who published it in 1978. The first book has a very few photos, there's more in 'Canal People'.
Whilst I have no evidence other than circumstantial that this is the same Harry Hanson, it all seems to fit - and it's even possible that the David & Charles money paid for Lyndon to be built in the first place. A nice thought.
Incidentally, Jim Shead's site lists two other boats built by John Else - I'd better make sure on getting the license next year I add that info on, and make it a third.