If my baby's bedroom in winter gets too cold for a 3.5 TOG sleeping bag (12-13 degrees), is it ok to put them in multiple sleeping bags eg 1 + a 3.5 TOG ? Or 2 x 2.5 TOG bags instead of using a heater? Thanks

It is not necessary to monitor the room temperature or to leave the heating or cooling on all night, as long as the baby is dressed appropriately for the room temperature.
Dress baby as you would dress yourself – comfortably warm, not hot or cold.
https://rednose.org.au/article/room-temperature

If you are concerned baby is cold, it may be necessary to add another layer of clothing under sleeping bag. More than one sleeping bags is not recommended.