Look in the archive posts for a very recent thread on this. It can be a problem if your cistern is filled from the mains but does the room have good ventilation as in extractor fan? You will only get condensation on the cold cistern if there is moisture in the air! In addition to the fan, other solutions are to connect the cistern to the cws tank; connect it to the hws pip ; live with it.
On the odd occasions I get it, I just live with it!