A 29-year-old woman presents with 7 weeks of amenorrhoea and intermittent lower abdominal pain. Her beta-hCG is 2500 IU/L, and transvaginal ultrasound shows no intrauterine gestational sac and a 2 cm adnexal mass. She is haemodynamically stable with no significant free fluid on ultrasound. What is the most appropriate management?