Experimental: Single treatment arm

All participants who are deemed eligible for inclusion in the study following screening will be enrolled into a single experimental arm which will comprise the following sequential stages:~1. Baseline sampling; participants only use 'as required' ipratropium bromide when needed (1 week)~2. Salmeterol xinafoate monotherapy 50 μg twice in the morning and twice in the evening + 'as required' ipratropium bromide when needed (2 weeks)~3. Washout period; participants only use 'as required' ipratropium bromide when needed (2 weeks)~4. Salmeterol xinafoate 50 μg / fluticasone propionate 250 μg combination therapy twice in the morning and twice in the evening + 'as required' ipratropium bromide when needed (2 weeks)