Guidelines on the management of asthma. Statement by the British Thoracic Society, the Brit. Paediatric Association, the Research Unit of the Royal College of Physicians of London, the King's Fund Centre, the National Asthma Campaign, the Royal College of General Practitioners, the General Practitioners in Asthma Group, the Brit. Assoc. of Accident and Emergency Medicine, and the Brit. Paediatric Respiratory Group