A 30 year old lady presents with a three-month history of episodes of headache. The headaches can start any time of the day and last hours, with associated nausea. They resolve after the patient goes to lie down in a dark room. There are no visual changes. The headaches are occurring around once per week and are causing the patient to miss days of work. The patient is usually fit and well. She takes paracetamol when the headache occurs which doesn’t help and takes microgynon 150/30 OD. On fundoscopy you see the following (see comments of post). The headaches settle with once-off doses of sumatriptan although she is having to use this two-three times/week. What treatment would you start?