Is there a bye-law that says that I need a bell on a towpath?

To be honest if you regularly use a towpath you should fit a bell, its just good manners really.

I use the towpath here in belfast and a quick ting just to let people know your there and to to move in to let you past keeps every one happy.

+1.
Well said.
Cyclists are on sufferance to use the towpath (by rights we should buy permits).

Inconsiderate behaviour may change this.