Last winter we used a s/s grosuit, a l/s grosuit and a 2.5 TOG bag, plus blankets! I know they say not to but I think you ave to look at each child individually. We also had a heater in her room that kept it to 16 degrees overnight and she was alsways warm when she woke up - never too hot or sweaty. Now shes older we don't have a heater in her room and Melbourne mornings have dropped down to 2-3 degrees this week! She is in a s/s jumpsuit, socks, a polar fleece Bonds jumpsuit, a 2.5 Snugtime bag, plus about 2 cotton blankets and one light fleecy one. Sounds like alot when I put it like that but she is never too hot - if anything the times she wakes is because she is cold from moving out from under the blankets.
I think it is a case of trial and error - good luck!