This study looks at the effects of weight loss in people who have heart failure with preserved ejection fraction (HFpEF) and are overweight or obese. The main questions it aims to answer are whether weight loss in this group of people improves: 1. The heart's shape, how well it pumps blood and how well it uses fuels 2. The person's quality of life and how much they can exercise Participants will attend 2 study visits, separated by 3-6 months. The intervention period takes place in between the 2 study visits. Each study visit will involve measurements including: * Symptom and quality of life questionnaires * Body measurements such as height and weight * Blood tests * Ultrasound scans of the heart (echocardiogram) * Magnetic Resonance Imaging (MRI) scans of the heart * Exercise components during the scans * 6 minute walk test Participants are randomly allocated (in other words, by lottery) to either 'diet' or 'control' groups. The control group will continue standard care. The diet group will be enrolled in a weight loss program supervised by the study team. Participants in the diet group will replace their usual meals with meal replacement products specifically designed to deliver a low calorie diet for weight loss. The products will consist of formula soups, shakes and porridges. This diet would last 8 weeks, followed by a guided period of food reintroduction and maintenance.