Aortic stenosis (AS) is caused by narrowing of one of the main heart valves. Replacing the valve is the only treatment to prevent the heart from failing or death. The timing of replacement is currently often too late - half of patients are left with permanent scarring and a quarter die within 3.5 years. Studies are underway to see if earlier replacement makes a difference. But for those with scarring of the heart, there is currently no tailored treatment. I want to change this by understanding why and how patients with scar are dying and what the investigators can do to prevent this. In this study, the investigators will use a heart scan (MRI) to detect scarring before valve replacement. After replacement, patients will receive a tiny monitor (paper clip size), which the investigators inject underneath the skin. This monitor continuously checks the heartbeat and can detect increased body fluid due to heart failure. The investigators will monitor patients for an average of 3 years to see if scarring is linked to abnormal heart rhythms and heart failure. Once the investigators know how and why, the investigators can target patients with available medications and design studies using specialised treatments, eg defibrillator implantation, to protect patients with scar from dying.