"done everything by the book" and she's under the sink??
I've been keeping and breeding tortoises for decades. Temperatures need to be as stable as possible and always between (absolute max and min) between 1C to 10C, but ideally 4C to 7C. Any warmer than 10C and she'll be using up vital fat reserves and/or dehydrating. Any cooler than 1C and freezing is the potential killer
Also, hibernation needs a wind down period beforehand i.e. gradual reduction in temperature while undertaking a period of fasting (the length of time with no food is dependent on weight/size) but with frequent drinks right up until they're put into hibernation. Without the fasting, undigested food in their stomach can rot and kill them.
Have you kept good weight/growth records over the year to be sure that she's healthy and has enough reserves to survive hibernation?
To explain a little, I'm in an old house, there is no heating in my kitchen, and the sink is in the same part of the kitchen as the larder (and the other end to the oven - its a long thin kitchen) backing right onto the back door. If anything, I'm worried about it getting too cold when the weather turns! Shes not in the larder because there is now a radiator on the back of that wall, so it is warmer than it was.
Checked her temps last night (evening, heating in the house was on) and shes fast asleep and the sand/soil is 5'
She has wound down food/temp wise and been bathed throughout (plus had water).
She hasnt been weighed through the year, but her weight is average according to the jackson (think that was it) ratio.
I'm paranoid because I'd been given awful advice on how to look after her by the man I got her from, and i only found this out last winter (when she wasnt hibernating). Its not a normal pet store, he's a snake breeder, and really is an expert with snakes. Not so much with the torts though.