The movie was based on Anne Lister's diaries which were discovered and decoded almost 150 years after her death. They were never published in her lifetime. That would have been scandalous!
The film was probably based on these two books.
Whitbread, Helena. I Know My Own Heart: The Diaries of Anne Lister 1791–1840. (Virago, 1988)
Whitbread, Helena. No Priest But Love: Excerpts from the Diaries of Anne Lister. (NYU Press, 1993)
"I love and only love the fairer sex and thus beloved by them in turn, my heart revolts from any love but theirs." - Anne Lister, Journals, Oct 29, 1820