A 68-year-old man with chronic kidney disease stage 4 (eGFR 25 ml/min/1.73m²) develops symptomatic anaemia with Hb 85 g/L. Ferritin is 150 mcg/L, Transferrin saturation is 25%, and B12/folate levels are normal. He is not actively bleeding. According to NICE guidelines, what is the most appropriate next step?