A 70-year-old man with benign prostatic hyperplasia (BPH) is taking tamsulosin 400mcg OD. He presents with ongoing troublesome lower urinary tract symptoms (LUTS). His IPSS score is 18. His prostate volume is estimated at 45mL on digital rectal examination. Post-void residual volume is 80mL. According to NICE CG97, what is the most appropriate next pharmacological step?