[Rojofo]

Well, JSWolf, I have downloaded and tested the font you suggested. Unfortunately the problem remains.
When I chose to change the font to the new font you suggested, in the "advanced" font menu on the Kobo, the font appears as having a large "weight" but when I reduce this on the menu and return to the book, the "bold" type remains unchanged.
Other ebooks on my Kobo do not have this problem, only a few.

My layman's impression is that these problematic ebooks have been formatted with a specific font included which appears as "bold" and I would like to remove this formatting using Calibre but I don't know how to do that.