Aortic stenosis is narrowing of the aortic valve, and is the commonest type of valve disease requiring surgery. Current guidelines recommend waiting till patients develop symptoms (chest pain, breathlessness and dizzy spells/fainting) before possible open heart surgery to replace the valve is offered. However, studies using detailed 'MRI' scanning of the heart have shown that up to half of the patients already have 'scarring' in the heart by the time symptoms develop. Furthermore, scarring is not fully reversed even after surgery and is associated with worse outcome. This suggests that some patients are being offered treatment too late. Two randomised trials in the UK (EASY AS and EVOLVED) are currently investigating if valve replacement before symptoms will result in better survival. The aim of this study is to compare the effect of early valve replacement versus waiting for symptoms, on the amount of scarring in the heart. The investigators want to know if early treatment leads to less overall scarring at the end, and leads to better quality of life and recovery after surgery. The investigators will invite participants of the EASY AS and EVOLVED trials to have 1-2 MRI heart scans: at recruitment and 3 years after being randomly allocated to early aortic valve replacement vs 'watchful waiting'. The investigators will also assess the impact of the two treatment approaches on quality of life, disability-free survival (using questionnaires) and recovery after surgery. The results from this project will increase understanding of the results of the main trials, and lead to improved selection of patients with aortic stenosis who are likely to benefit from early surgery.