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My house is about 5 years old. In my loft are two plastic tanks - a massive round one and a tiny box one. We have a big cylinder in the airing cupboard, which heats the hot water on a set timer (not on demand). We have a control for the hot water, and another for the central heating.

I'm trying to identify if one of them holds water ready to go into the cylinder to heat the central heating/water, so I can put a descaler in there.

What are the two tanks for?

SMALLER OF THE TWO WILL BE THE CENTRAL HEATING FEED & EXPANSION TANK.

BIG ONE WILL BE MAINS COLD WATER STORAGE TANK.

big one is your cold water storage tank which will feed the hot water cylinder in your airing cupboard. When you open a hot tap the pressure change allows cold water from this tank into the bottom of the hot water cylinder efeectively pushing the hot water out of the cylinder at the top and to your taps. This CW tank may also provide the cold water to your toilet and bathroom cold tap (but these may also be mains fed)

The smaller tank is your feed and expansion tank for your central heating system.

The cylinder in your airing cupboard has nothing to do with your central heating and is for hot water only. If it is an indirect cylinder there will be a sealed coil shaped like a spring inside, through which hot water from your boiler and central heating will pass through - this hot water will heat your water inside the tank by emitting heat through the coil pipes and thus 'indirectly' heating the water inside the tank (it never touches/mixes with this water, but heats via convection through the coil.

In addition to this you may also have an electric immersion element that sits at the top of your cylinder providing a back up heating source for your hot water.