STEWARDSONS
Fishaholic
Depending on what type of boiler system you have at home you can bring the new water up to about the right temp by adding hot water from the tap. If you have an old emersion type heater system then it's best not to do this (especially if it's one of those up in the loft or on the roof as it's quite common to find dead things floating about in them
The tank you find in the loft with dead stuff in is the central heating header/expansion tank and isnt connected to the hot water tap. Hot water for the tap is stored in a insulated copper cylinder usually in the airing cupboard. The reason you shouldnt drink from the hot tap is that this cylinder is a prime breeding ground for bacteria as the hot water cools down, which is then flushed out the tap the next time you use it.
I have a hot water storage tank but for the tank upstairs I use the electic shower in the bathroom to temp match the water as the water is heated as it passes through the unit. Downstairs I find that half a hot kettle per 5 gallon bucket of cold water gets it to the right temp.
i always use the mains taps int he kitchen, a bit of the hot and a lot of the cold, then i put the thermometer in to check temp