Ventricular tachycardia (VT) is an abnormal rhythm arising from the bottom chambers (ventricles) of the heart. The hearts of most patients who develop VT have been previously damaged by a myocardial infarction (heart attack) or other heart muscle diseases (cardiomyopathies). The damage produces scar or fatty deposits that conduct electrical impulses slowly allowing VT to occur. Recurrent episodes of VT can compromise heart function and increase mortality. VT is prevented by special drugs but these are not always effective and can have many side effects. Most patients with VT will also have a specialised device called an implantable defibrillator (ICD) implanted. The ICD treats VT by either stimulating the heart rapidly or delivering a shock to it. ICDs are very effective but the shocks are painful and have a big impact on quality of life. If VT occurs despite optimal drug treatment, patients undergo an invasive procedure called catheter ablation. Here, wires are passed into the heart from the blood vessels in the leg and the damaged heart muscle causing the VT is identified whilst the heart is in VT. An electrical current is passed down the wire making its tip heat up allowing discrete burns (ablation) to be placed inside the heart. The ablated heart muscle doesn't conduct electricity which stops the VT and prevents it recurring. Some patients are so frail that ablation cannot be performed safely. A recent clinical trial has shown that VT can be treated in such patients using radiotherapy, which is usually used to treat tumours with high energy radiation. This approach is non-invasive, painless and requires no sedation or anaesthesia. This study will test whether VT can be successfully treated using stereotactic ablative radiotherapy. This can deliver high dose radiotherapy very precisely, whilst minimising the risk of damage to healthy tissues.