I never thought of Diana Dors as a serious actress, more like a dumb blonde a la Jane Mansfield. (She was, in fact, billed as “the British Marilyn Monroe.”) On Sunday, at Film Forum, I saw Dors in “Yield to the Night,” a British crime drama directed by J. Lee Thompson in 1956, which made me rethink my opinion of the actress. She’s surprisingly impressive as a woman waiting in prison to be hung for killing the other female in a love triangle. Her understated performance might bring a tear or two to the eyes of even the most vocal advocate for capital punishment. Supporting performances are also effective. The story is loosely based (like “Dance With a Stranger”) on the infamous real-life case of Ruth Ellis, who in 1955 became the last woman to be executed in the UK. (She had murdered her lover.)