Looking for story--denial in a harem

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I am looking for a story that had a british woman kidnapped and sold to a sultan's harem, where the sultan "owns" the orgasms of the women. They wear a magic gold anklet that determines if they climax and if they do they are severely punished. They must masturbate to the brink of orgasm every night, and the sultan picks the one who is closest to cumming to be his bedmate.

Any ideas?