First of all, so sorry for your loss @Luckycatyellowsky
But in response to @PotatoScollop
"having plenty of objects around the basking spot (and all over the enclosure to be honest) so they can right themselves when they tip over, is vitally, crucially important."
As a tortoise breeder of 30 years experience, I strongly disagree with your advice about having plenty of objects positioned around the basking spot. That is actually the number one cause of this sort of tragedy, along with having a basking spot that is far too hot.
There is a greatly reduced likelihood of the tortoise flipping onto its back under a heat lamp if that basking spot is well away from any obstacles. Any tortoise is far more likely to flip when it's climbing over or attempting to scale something. Their heat lamp needs to be positioned well away from the side walls of the enclosure as well as away from obstacles within their habitat.
Yes, having things around to push against helps them to right themselves, but there's no guarantee even then that they will be able to do so, so the main aim is to make it less likely that they will flip over in a dangerous position.
I also always advise people who take my hatchlings that it is FAR safer for them to be exposed to cool temperatures for a while than it is to be left unsupervised with a heat lamp set to provide a basking spot of ~30C, so I recommend that if they're leaving them unattended for any length of time, they turn off the heat lamp (or raise it high enough to give a ~20C basking spot) and just leave the hatchling at room temperature while they're out.