I've not got much room in the house for demijohns so am trying keeping them in the allotment shed over the Winter. This is mostly of brick construction with a concrete floor. Any ideas how to keep the fermentation going over the Winter?
Isn't it supposed to be kept between 60 and 70 degrees F. (forget what that is in Celsius)
I'd find somebody with a basement.
you could try wrapping them with fleece and bubble wrap....personally id be tended to ask a friend to store them as the little scrotes around here would smell out free alcohol from 5 miles away...