A 75-year-year-old man with a history of stable angina is prescribed simvastatin 40mg ON. He is also taking amlodipine 10mg OD for hypertension. Four weeks later, he complains of new-onset muscle aches and weakness, particularly in his thighs. His GP checks his creatine kinase (CK) level, which is found to be 1500 U/L (normal <190 U/L). What is the most appropriate immediate action?