See the section on the writing process of "Hogen Monogatari" to learn more about the writing process and authorship; there are many overlaps between Hogen Monogatari and Heiji Monogatari, particularly in regard to the writing process and authorship.
なお、成立・作者にかんしては『保元物語』の保元物語成立・保元物語作者と重なる点がおおいので、そちらも参照されたい。