It will probably fit ok but you may need new mounting holes if they don't line up, not a big issue really just fill any old holes not used. For river use they can be a pita in locks though, may be better getting a single central pilot rail made so you get a handhold without the trip hazard.
Thanks for your reply, df. I remember your mention of a single handrail sometime around last October and remember your mention of the annoyance of the rope going over the pulpit rail when you want it under, and vice versa! Hopefully, this will be less of a problem on canals. My boat's original rail was removed by the previous owner, and the mounting holes were filled and refinished leaving no trace. I'm fairly confident that this rail is going to fit, but I'll ask for some measurements to be sure.