The investigators have demonstrated that they can reliably identify an optimum heart rate range for contractility of the left ventricle in patients with chronic heart failure (CHF). They have also demonstrated in an acute cross-over and a small parallel group feasibility study that keeping the heart rate in this range (versus standard rate-response programming) in patients with CHF is associated with increased exercise time on a treadmill (around 60s or 10%). They now want to explore in a randomised, placebo-controlled 3-arm parallel group trial whether optimal programming versus standard rate-response programming versus no rate-response programming for 6 months leads to appreciable improvements in exercise time and quality of life, while having no adverse effects on left ventricular function and battery longevity and what the mechanisms of this might be. 450 patients with CHF and a pacemaker will undergo the non-invasive echocardiographic assessment to establish the force frequency relationship and the optimal heart rate for contractility. They will then perform a treadmill walk test, complete quality of life questionnaires and be offered the opportunity to participate in a series of mechanistic substudies. They will then be randomised to optimal rate-response settings, standard rate response settings or no rate-response settings and followed up at 6 months at which point the tests will be repeated.