 Design The investigators have chosen a randomised crossover design to answer this question because it is unknown which, if either, beta-blocker might be better for lung function and exercise capacity in moderate to severe COPD patients. The crossover design allows the use of the same patients as their own comparators, thus allowing the investigators to expose fewer patients to these medications with the same statistical power as a parallel group study different patients in each group. Each participant will therefore receive both treatments in sequence but with their initial one decided at random using a randomisation algorithm. Importantly the investigators have not used a placebo control arm here, as it is already known that beta-blockers are of benefit to COPD patients particularly those with cardiovascular disease whereas placebo provides no benefit. In other words the investigators are drilling down to see which particular medication has the most beneficial therapeutic profile. The justification for using open-label beta-blockers is that this is a pilot study and it helps to keep the overall costs of the study down at this stage of investigation. 