To keep the nation's pictures safe from German bombs in 1939, the National Gallery evacuated its entire collection from Trafalgar Square to the Manod slate quarry near Blaenau Ffestiniog, Wales. Emrys Evans recalls seeing the lorries lumbering up the valley to their secret destination over 60 years ago, and current gallery staff wonder how they would react to the threat of war today. Producer Lucy Lunt