Inclusion Criteria:  Diagnosis of COPD as determined by a post bronchodilator FEV1/FVC <70% and an FEV1 of between 20 and 80% at screening visit Treatment with inhaled triple therapy (licensed combination of long acting beta 2 agonist, long acting anti-muscarinic and corticosteroid) at constant dose for at least 12 weeks before screening visit. Treatment with roflumilast, theophyillines and macrolides will be permitted so long as they were introduced at stable dose > 12 weeks prior to screening visit. (If maintenance drug dosing has not been with stable dosages for 12 weeks the screening visit may be rescheduled until this is achieved: see sections 7.3 and 7.10) At least 2 acute exacerbations of COPD (AECOPD) requiring treatment with oral steroids and/or antibiotics in the last 12 months, or 1 acute AECOPD requiring hospital admission in the last 12 months. At least one eosinophil count of >0.3 cells·μL-1 in the 12 months prior to screening Age over 18 years  Exclusion Criteria:  Contraindication to MRI scanning, including Gadovist (ie hypersensitivity or poor renal function; see below); this includes claustrophobia and musculoskeletal difficulties, this information is collected on the UoS MRI unit screening form. Inability to give informed consent or comply with study procedures Hypersensitivity to mepolizumab or its excipients Untreated helminthic infection Exacerbation of COPD requiring treatment with oral steroids and/or antibiotics within 4 weeks of screening. A repeat screening visit may be scheduled in order to achieve this criterion. The participant will be required to successfully complete all screening procedures at the rescheduled visit, including that for exacerbation-free stability. SpO2 <90% on room air at screening Clear history of childhood and/or current asthma Past history of lung surgery Other significant lung disease Long term oral steroid treatment eGFR < 30 ml/min/1.73 m2 at screening NYHA class 3 or 4, where the functional limitation from heart disease is greater than that from COPD, or uncompensated heart failure Chronic liver disease (Any elevation of ALT above twice the upper limit of normal at screening. Lower levels of abnormality are permitted after investigator review if felt not to compromise safety) Malignancy unless treated and disease free for 5 years Conditions causing significant immunosuppression Active infection with blood borne viruses (including hepatitis A and B and HIV) Other significant medical condition compromising participant safety or fidelity of study. Pregnant or breast feeding Of childbearing potential and not willing to use highly effective methods of contraception during the course of the study and for 100 days post last dose of mepolizumab. Participants who have received an investigational drug within 30 days of first dose, or within 5 drug half-lives of the investigational drug, whichever is longer