I'm in Sydney and we don't run the heater overnight. Its get to the single figures in temperature. On our bed we have a very heavy wool doona and electric blanket so I don't think a 2.5 tog sleeping bag is enough for Oliver. I currently have for him:

Singlet
Short sleeve bodysuit
Wondersuit
2.5 tog sleeping bag (sleeveless)
sheet
fleece blanket

And in the last week I've added the cot quilt as well as he was waking again (yes, I know, but its tucked in at the foot of the cot like the sheet and blanket so I fail to see how it can be a greater risk than if I were to pile 2-3 loose blankets on top). Oliver sleeps with his arms up so I think he looses a lot of heat that way - he always has icecubes for hands in the mornings.