I've got a Vokera Compact combi boiler (12 months old) installed in a first floor room of my house. It's cracking at central heating (quiet, heats up dead fast) but theres a problem with the hot water. When hot water is run from any tap in the house it fluctuates between napalm hot & luke warm. As we have a simple mixer shower head with the hot feed from our fluctuating combi and cold directly form the mains it makes an early morning shower a bit punishing . I live at the bottom of a valley with pretty high water pressure. Please help so that I can avoid joining a local gym to use their showers.