Brian Dawson has done a lot of extra research on this and has come up with some astounding facts. The following is off the top of my head as he told me about it about 18 months ago.

During WWI the Delft family moved from Grimsby to Lowestoft and changed their name to something more English-sounding, because of their perceived or a real threat from locals at their German-sounding name (actually Dutch). Some time soon after this Wm Delft's grandson joined the Lowestoft Fishermen's Choir and took his grandfather's song and altered it so the choir could sing it, adding in the chorus.
Fishermen from different towns and indeed their choirs naturally meet up with each other and so a jump from Lowestoft to Filey is not a great one in terms of passing on sea songs.

Brian actually got this information from one of the family still alive in the Lowestoft area.