well i got all the info and went to do it. changed the pond water and turned heating off. loaded pond with autumn leaves and barley straw in their sleeping area.
temp slowly climbed down to 11 celcius (need under 8 to hibernate) but the pond got stinkier and stinkier as you not meant to change the water once you start the hibernation. the fish were gasping for air and the apple snails were close to death and covered in slime.
we got a warm snap in the weather and the pond was going up and down in temp and was so smelly i gave in and washed them and put them in the tank, they now adjusting but seem happy and we slowly got the temp up to 22 celcius.
i had fab advice from a rescue centre close to my town who hibernates 30 odd turtles at a time but i couldnt stand the smell and the antisocialness of it all so. we wouldnt see them for days on end.
But I wouldn't bother with having your turtles hibernate anyways. They're more fun swimming around.