A 48-year-old woman with newly diagnosed primary hypercholesterolaemia (total cholesterol 320 mg/dL, LDL-C 210 mg/dL, HDL-C 42 mg/dL, triglycerides 180 mg/dL) is started on simvastatin 20 mg daily. After 6 weeks, repeat lipids show LDL-C 165 mg/dL (21% reduction). She reports mild muscle aches but no creatine kinase elevation. What is the most appropriate next step?