Hi WillR,
As I am sure you are aware, you have posted to the forum of the IGKT (International Guild of Knot Tyers) and one of our publications is the "The DIY Book of Fenders" by Colin Jones. It is here:
http://www.igkt.net/publications/ . I understand it addresses the particular fenders used by narrowboats (I do not own a copy but, it is on my "one day" list

). In addition, Des Pawson, an IGKT member, shows fenders in his Knot Craft book.
A picture of Dave Walker, the IGKT Hon. Secretary, in one of the "Knotting Matters", a quarterly publication of the IGKT, shows him standing beside a large fender in his workshop. You have posted to the right place to look for information on fenders for narrowboats. I am confident others will point you in the right direction. I live in Canada, not the U.K., and only enjoy pictures of the narrowboat!
Cheers, Brian.
Ps. wear gloves
