Inclusion Criteria:  Clinical diagnosis of heart failure Plasma NT-proBNP >125ng/L or BNP >35 ng/l Left ventricular ejection fraction (LVEF) <50% on prior imaging test Treated routinely with a daily dose of loop diuretic Receiving other guideline-indicated therapy for heart failure Willing to sign the informed consent form Greater than 18 years of age  Exclusion Criteria:  Chronic Kidney Disease Stage 4 or worse (eGFR <30 mL/min/1.73 m²) Atrial fibrillation Significant valve disease (investigators opinion) Breathlessness or chest pain at rest or minor exertion Patients unable to lie flat Patients deemed too unstable to miss treatment with diuretics for 48 hours Patients taking part in another interventional trial Any other concomitant condition that in the opinion of the investigator would not allow a safe participation in the study (ie:-pregnancy, significant frailty) Patients unwilling or unable to consent For patients willing to undergo MRI; contraindication to MRI including claustrophobia, metalworkers, and intra-orbital or intracranial metal (eg:- stents), and non-MRI compliant device.