The measures of exercise capacity at the follow up assessment will be carried out off oxygen and will be carried out by an observer blinded as to whether they received oxygen or not during PR. All patients will be provided with ambulatory oxygen for domiciliary use after completing PR. All patients will have repeat assessments at 8 and 12 weeks after completion of PR. At these visits data will also be collected on activity, number of exacerbations and hospitalisations and ongoing usage of ambulatory oxygen.