Heart failure with preserved ejection fraction (HFpEF) is a common and serious complication of obesity and type 2 diabetes (T2D). HFpEF occurs when the heart muscle unable to relax efficiently to pump the blood around the body. This leads to fluid build-up, breathlessness and inability to tolerate physical exertion. People who develop HFpEF do less well because treatment options are limited. Pilot data in patients with obesity and diabetes and a small number of patients with HFpEF have shown improvements in exercise capacity and reversal of changes in the heart and blood vessels. This study will assess if this is achievable in a multi-ethnic cohort of patients with established HFpEF. A total of 63 adults will be invited and allocate by chance into two groups: 1) 12-weeks of a low calorie diet or 2) Standard care and health advice on how to lose weight followed by the option to have the low calorie diet after 12-weeks. The study will determine if weight loss over 12 weeks can improve heart function, symptoms and ability to exercise. Additionally, participants' views on changing their diet and how this has impacted their symptoms will be sought during the study in an optional interview. This will help guide treatments planning in the future to get maximum benefits, and to individualize support to patients from different cultural backgrounds.