Spirometry (FEV1, FVC), reversibility testing: Baseline pre-bronchodilator spirometric measurements will be recorded from the best of three attempts using a dry wedge spirometer (Vitalograph, Buckingham, UK) with measurements not varying by more than 5% or 0.2ml. Spirometric measurements will be made before and after nebulised salbutamol (2.5mg). Measurements will be performed at the same time of day (am or pm) for each patient.