A 29-year-old woman presents with 6 weeks of amenorrhoea and lower abdominal pain. Her serum β-hCG is 1 800 IU/L. Transvaginal ultrasound shows no intrauterine pregnancy and a 2 cm adnexal mass without free fluid. She is haemodynamically stable, has no significant pain and reliable follow-up. Which is the most appropriate management?