Comparing Three Types of Specialist Pacemakers to Improve Heart Function and Reduce Rhythm Problems in Heart Failure

The goal of this clinical trial is to find out which type of specialist pacemaker-known as cardiac resynchronisation therapy (CRT)-works best for people with heart failure and a delay in how the lower chambers of the heart beat together (called electrical dyssynchrony). The main aims of the study are: To compare the effects of conventional biventricular pacing (BVP), conduction system pacing (CSP) and left-bundle optimised CRT (LOT-CRT) on heart failure symptoms and heart rhythm problems over six months. To explore how these pacing methods affect heart muscle strength, electrical activity, and overall heart function. Participants will: Attend four hospital visits over a six-month period. At Visit 1, meet a member of the research team to discuss the study and have screening tests to check eligibility. Participants will also have a smartphone app installed and receive training on how to record their daily heart failure symptoms. At Visit 2, have a CRT pacemaker implanted. The type of pacemaker will be chosen at random, with a 1 in 3 chance of receiving: * Biventricular pacing (BVP); the current standard treatment * Conduction system pacing (CSP) * LOT-CRT (Left-bundle optimised CRT); a combination of both At Visit 3 (around 12 weeks after implantation) and Visit 4 (6 months after implantation), take part in routine follow-up assessments to check the pacemaker and heart function. At Visits 2 and 4, also undergo non-invasive electrical mapping tests, including wearing a specialised vest and having a low-dose CT scan of the chest. These tests help researchers understand how the heart's electrical system responds to different pacing methods.