Upregulating the Nitric Oxide Pathway To Restore Autonomic Phenotype (UNTRAP).

Inclusion Criteria: * Participant is willing and able to give informed consent for participation in the study. * Male or Female. * Diagnosed with chronic heart failure - NYHA II-III * Reduced heart rate variability * Left ventricle ejection fraction (LVEF) of ≤40% * Sinus rhythm on 12 lead ECG * Must have an adequate understanding of written and spoken English * Female participants of child bearing potential must be willing to ensure that they or their partner use effective contraception during the study and for 3 months thereafter * Able (in the Investigators opinion) and willing to comply with all study requirements Non-Invasive Programmed Stimulation (NIPS) Sub-Study Inclusion Criteria: * Participant is willing and able to give informed consent for participation in the study * Patients has a pre-existing ICD device with right ventricular apical lead Exclusion Criteria: * Myocardial infarction or coronary revascularization within the last 6 months before study enrolment * NYHA class IV heart failure symptoms * Persistent Atrial Fibrillation (AF)/Atrial flutter, or paroxysmal AF with frequent recent episodes of prolonged AF * Patients taking any other nitrate containing medication or supplement (e.g. Isosorbide mononitrate, GTN) * Patients taking proton pump inhibitors * Severe pulmonary disease * Significant renal impairment (eGFR\<15) * Active cancer with life expectancy \< 1year * Patients with significant diabetic or other autonomic neuropathy * Current or recent (w…