A 60-year-old man is reviewed in the respiratory clinic for his Chronic Obstructive Pulmonary Disease (COPD). He uses Salbutamol PRN and Tiotropium inhaler once daily. Despite this, he remains breathless on moderate exertion (MRC dyspnoea scale grade 3) and has had 2 exacerbations requiring oral steroids in the past year. His FEV1 is 45% predicted. According to NICE guidelines, what is the most appropriate next step in his inhaler therapy?