A 50-year-old woman with known stable asthma, managed with salbutamol inhaler as needed and a low-dose inhaled corticosteroid (ICS) twice daily, reports using her salbutamol inhaler 4-5 times per week and waking with wheeze once a week over the past month. Her peak expiratory flow rate (PEFR) is 85% of her best. According to BTS/SIGN guidelines, what is the most appropriate next step in her asthma management?