A 40-year-old woman with severe persistent asthma uses a Salmeterol/Fluticasone combination inhaler (e.g., Seretide 250 Accuhaler) twice daily and Salbutamol as required. She still experiences symptoms requiring her reliever inhaler 4-5 times most days. Her inhaler technique is good. According to BTS/SIGN guidelines, what is the next most appropriate step in her asthma management?