Recurrent wheezing and asthma practice

A 15-year-old girl attends a general practice for a repeat of her salbutamol inhaler. She has had asthma since childhood. She uses salbutamol two or three times in most weeks, once or twice a month at night, and always before netball. She has not used a preventer for two years. Her inhaler technique is good and she has had no flare-up needing oral corticosteroids in the past year. Spirometry shows an FEV1 of 84% predicted, rising to 95% after salbutamol. Which one of the following is the most appropriate ongoing treatment?