I have a system boiler with 2 unvented hot water cylinders. There is no hot water in the loft bathroom tap. But hot water is coming in the loft bathroom thermostatic shower. Moreover hot water is coming in the other 8 bathrooms. When you switch on the loft bathroom shower, hot water starts coming in the loft bathroom tap. But on its own the loft bathroom tap is giving cold water. Any suggestions would be greatly appreciated. Two plumber cum gas care engineers have already given up. I have installed a new tap and a new thermostatic shower in the loft bathroom to eliminate the problem and come to a diagnosis. But no luck so far.