A 65-year-old man with known COPD (FEV1 45% predicted, 2 exacerbations in past year) presents for annual review. He is using salbutamol PRN and a combination inhaler (ICS/LABA - e.g., Seretide 250 evohaler BD). Despite this, he remains breathless on minimal exertion (MRC dyspnoea scale 4). According to NICE NG115, what is the most appropriate next step in his inhaler therapy?