A 68-year-old woman with a history of myocardial infarction 6 months ago presents with recurrent episodes of sustained monomorphic ventricular tachycardia (VT). She is haemodynamically stable during the arrhythmia. Baseline ECG shows evidence of prior anterior wall MI with reduced ejection fraction (EF 35%). Before initiating long-term antiarrhythmic therapy, which investigation is most appropriate to assess the substrate and guide treatment decisions?