Have now found a couple of places where Hammy sleeps in her tunnel purely by chance. While my hands were touching the base of her cage as I was looking into it earlier, I noticed one spot was warm. Wondering why, I opened the gate and gently picked a small amount of bedding off. Just then, who should make an appearance, none other than Hammy herself. I've just found another spot where it is warm, but i won't disturb her this time. It seems she is sleeping up against the base of the cage, and the heat of her body is what I can feel seeping through.

Not surprised shes down there in this weather. I think I would be as well.