Pilot study work has shown that the work is technically feasible and safe. The initial data has been published in a highly regarded scientific journal. This main study will be a multicentre randomised controlled trial whereby patients will be randomised to either standard treatment or pressure wireguided treatment. The patients in the pressurewire guided arm will have a specialised wire implanted into the main pumping chamber of their heart via a blood vessel at the top of the leg (or less commonly a blood vessel in the wrist). This will measure acute pressure changes over time and the pacing lead will be positioned in the site that gives the greatest change in pressure.