A 33-year-old woman at 7 weeks gestation presents with lower abdominal pain and light vaginal bleeding. Her serum beta-hCG is 1500 IU/L and transvaginal ultrasound shows no intrauterine pregnancy and a 2.5 cm unruptured tubal mass with no fetal heartbeat. She is haemodynamically stable and reliable for follow-up. What is the most appropriate management?