Heart failure patients may benefit from having a special pacemaker implanted that can make the heart pump in a more coordinated and efficient way Cardiac Resynchronisation Therapy (CRT). For the heart to pump well it is necessary for the pacing wires to be placed in optimal positions. Only two thirds of people respond to CRT and this may be because of non-ideal pacing wire position. A potential marker for response is the change in heart pump pressure change over time. The aim of this study is to use a specialised wire in the main pumping chamber of the heart to record the pressure changes with the pacing lead in various positions to determine the optimal position. It is postulated that the improved pressure changes will increase the proportion of responders.