A 28-year-old woman presents with a 6-week history of persistent dry cough, particularly troublesome at night. She has no fever, wheeze, or shortness of breath. She has a history of mild intermittent asthma, usually well-controlled with Salbutamol inhaler PRN. Examination and chest X-ray are normal. According to NICE guidelines on chronic cough, what is the most appropriate initial investigation or management step?