Your problem is the shower head. The Salamander pump that you have installed can pump approximately two gallons a minute at 2 bar pressure. However, the massive shower head that you have installed offers very little resistance to the water flow, and your pump is capable of pumping thirteen gallons a minute at 1 bar pressure, this is the reason that you are using up the hot water so quickly. If you install a more restrictive shower head the pump will not be able to pump such a huge volume of water in the time that you shower, and the hot water will last longer. BB