I could not find any evidence of her having won an award for that book.
http://www.ac.wwu.edu/~stephan/Renault/ ... .biog.html
In 1947, she won the MGM award of $150,000 (over £37,000 at the time) for her novel Return to Night. The MGM award was given to successful novels in hopes that they would make successful movies. The only drawback was that none of the winning books was ever filmed and the awards were abandoned shortly after Mary won her prize.