A 34-year-old woman with known asthma, well-controlled on Salmeterol/Fluticasone (Seretide) 250 Evohaler one puff BD and salbutamol PRN, presents for her annual asthma review. She reports using her salbutamol inhaler 4-5 times a week and has woken with wheeze twice in the last month. Her current PEFR is 85% of her best. What is the most appropriate next step in her asthma management according to BTS/SIGN guidelines?