A 24-year-old woman with known asthma, usually well-controlled on a salbutamol inhaler as required and a low-dose inhaled corticosteroid (ICS), reports using her salbutamol inhaler 4-5 times a day for the past week. She also wakes up coughing twice a week. Her peak expiratory flow rate (PEFR) is 75% of her best. What is the most appropriate next step in her asthma management according to BTS/SIGN guidelines?