Hello Edadam
I presume you are referring to the overflow from the hot water storage cylinder (in your hotpress) being connected to the (bigger?) cold water storage tank in the attic. Your situation sounds perfectly normal. The bigger tank is supplying cold water directly to the storage cylinder in the hot press and to the cold taps in your bathroom, wc cistern etc. This tank is in continuous use and is thus being partially drained and refilled on a regular basis. Thus the water will be clear as it is not stagnant and shouldn't warm up too much in the attic space.
The other tank (smaller) is a cold feed and expansion tank specifically for the central heating system and nothing else. The water in this tank (under normal operating conditions) is generally stagnant, except for some increase/ drop in level which may occur when the systems is heating up/ cooling down. The "scale " you refer to shouldn't alarm you. you might want to consider adding a corossion inhibitor to this tank (after partially draining tank) which minimise the occurance of any "scale" bacteria in the tank. This subject was covered in depth a month ago, under central heating problems.