If the body was luke-warmish,,it can be as near as dammit using a rectal thermometer ( warmest area )
I think they measure the temperature in the liver - or the brain. Those two organs because of their mass and density.
I wouldn´t want a thermometer stuck into my brain!!!!
There are so many factors, such as amount of body fat, room temperature, amount/type of clothing worn, amount of blood loss etc.
It is just hard to determine precicely, especially when the victims are found after some hours. I´d say though, that Sheila hadn´t been dead for all that long when she was discovered - but I am judging from unclear photos and the fact that rigor mortis had not set in at 10 am (ca: when the photographs were taken, to lazy to look the time up, don´t remember.)