Cardiac resynchronisation therapy (CRT) is an established treatment for improving symptoms in patients with congestive heart failure (CHF) by left ventricular (LV) pacing. CRT can help to improve LV function in patients with heart failure if those regions of the myocardium which are most compromised by electromechanical dyssynchrony can be identified and effectively stimulated. There still remains, however, a significant rate of up to 30% of patients who do not respond to treatment. Reasons for lack of benefit can be related to the inability of identifying and effectively stimulating those regions of myocardium, which are most compromised by electromechanical dyssynchrony. The investigators hypothesize that by using cardiac MR and 3D echo to identify scar, reconstruct coronary sinus anatomy, and determine the site of latest LV activation, the investigators can find the best place to implant the left ventricular lead. By avoiding scar and pacing in the site of latest activation, the investigators believe the investigators will reduce dyssynchrony and thus improve overall heart function. The researchers thus aim to increase the proportion of people who respond to treatment. The researchers also believe that the procedure may be streamlined so as to reduce procedure duration, radiation dose and dose of iodinated contrast medium.

Inclusion Criteria:

• >18yrs of age
• Standard indication for CRT=P or CRT=D according to NICE criteria (Clinical symptoms of heart failure despite medical therapy, broad QRS >120ms and significant LV dysfunction LEF <35%)
• Stable on optimal medical therapy for at least 3 months
• No exclusion to pacing /ICD
• Ischaemic or non-ischaemic aetiology

Exclusion Criteria:

• Any contraindication to pacing /ICD implant
• Contraindication to MR scanning
• Claustrophobia
• Significant renal impairment (estimated GFR <30)