We have the same problem, but only in the summer. If, by cistern, you mean the water tank that feeds the toilet (we're in the U.S.) then drain it, dry it thoroughly and line it with a plastic foam kit available from most DIY stores. Takes time, but does work.
Alternately, if you're a pretty adept DIY'er, install an antisweat valve in the water-supply line leading to the toilet. The valve adds a little hot water to the line, which raises the water temperature in the toilet enough to warm up the tank and bowl. You can hire it done as well... costs about $100 to have it installed here in the U.S.
Lastly, consider replacing your current toilet with a restricted flush new one. Here, the new ones hold about 1.6 gallons and, due to the smaller amount of cold water, they usually don't sweat as bad, if at all...