A 55-year-old man presents with a 6-week history of persistent dry cough, particularly worse at night and after exposure to cold air. He recently recovered from an upper respiratory tract infection. He does not smoke and has no history of asthma or GORD. Chest examination is unremarkable. A chest X-ray is normal. According to NICE NG10 (Cough - Acute), what is the most likely diagnosis?