It’s dead simple, get a washing up bowl and add really cold water enough to cover the bottom of the demijohn, about a couple of inches. Leave it in there for a while to make sure the base is nice and cold. Then just boil the kettle and pour in the boiling water. If your lucky it should crack instantly and the bottom just drop out. If not give it a twist.
There you have it instant mini green house, the best part is you recycle something thats otherwise useless and it won’t blow away like plastic bottles.