A 26 year old woman presents with 8 weeks of amenorrhoea and lower abdominal pain. Her last menstrual period is uncertain. Serum β hCG is 1500 IU/L and transvaginal ultrasound does not visualise an intrauterine pregnancy but shows a small adnexal mass and free fluid in the pouch of Douglas. She is haemodynamically stable. What is the most likely diagnosis?