I have been doing mine for years with no problems, I tell people to stop feeding at the end of the financial year(31st march) and resume on labour weekend,you can extend this time with experience but it is a good general rule of thumb to follow.As already said your turts must be in good health and condition beforehand,once the temps drop below 18 digestion is slowed and any further drop can stop it with food left in the stomach to rot and kill the turtle a result.The other thing is just leave them alone,you can`t be picking them up checking on them etc,trust nature to work,it does quite well and has for a long time.I used to check mine at night with a torch to see if they were still alive but nowdays are confident to leave them to it.They still come out to bask on sunny days and the males are alot more active than the females,some seem to go deeper into than others and you never see them even on a hot day.