Hi, My baby is 10 weeks and at night she is currently wearing a 2.5 Tog sleeping bag with a onesie and singlet. I also have a blanket over her tucked in when she is sleeping but when I feel the back of her neck it is cool. Does she need more layers or a higher TOG?

It can be difficult to work out what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rather than trying to modify the room temperature with heating or cooling.
Use light weight bedding & clothing & lightweight muslin or cotton for wraps.
Is she in bassinet or cot?

I wish I could give you some specific guidance for dressing a newborn, but every baby is different.
Red Nose has the following information.

Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.

Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.  - Always sleep baby with head & face uncovered.

A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal).
Whether the baby has a cold or infection or another special need.
Consider how many layers that you as the baby’s carer are wearing comfortably.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

A good way to check baby’s temperature is to feel baby’s chest or back of neck, which should feel warm (don’t worry if baby’s hands and feet feel cool, this is normal).
Ensure baby’s head is uncovered - no hats, bonnets, beanies or hooded clothing.
https://rednose.org.au/article/how-much-bedding-does-baby-need

As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.

We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.
https://rednose.org.au/article/how-to-dress-baby-for-sleep
https://rednose.org.au/article/how-to-dress-baby-for-sleep
https://rednose.org.au/article/protecting-baby-from-overheating-during-sleep