Inclusion Criteria:  A doctor's diagnosis of COPD PtCO2 ≥45mmHg on transcutaneous monitor at baseline measure Oxygen saturation measured by pulse oximetry (SpO2) ≥85% Fits the study mask Can tolerate the therapy  Exclusion Criteria:  FEV1/FVC ≥70% Body mass index >35 Hypercapnia considered to be primarily due to Obesity Hypoventilation syndrome and/or Obstructive Sleep Apnea Any condition which makes NIV contra-indicated as per BTS guidelines based on clinical judgement