In general, the solution to condensation is ventilation. Heating, whilst inside, makes it more convivial in winter, but it's not the low temperatures that harm stuff, it's the lack of moving air to dry the place out.
If there is any valuable stuff stored in there, a camera would be a useful accessory to have. In fact, I'd recommend two, with both of them feeding a remote recorder (in the house, or on t'web..?). This might be able to provide the constabulary with evidence, in case of infraction.
Just my tuppence-worth.