Hi, my baby will be born in Winter and I was contemplating buying a newborn sleep sack for the warmth (2.5 tog). I used a muslin wrap on my first but he was a Spring baby, I figure the sleep sack will be a warmer option but I'm unsure if it will be the safer option? My first born always seemed to undo his muslin wrap during the night so that was another reason for using the sleep sack. Any advice would be appreciated.

Red Nose recommends using a lightweight cotton or Muslin wrap if you are using any wrap/swaddle.
When you refer to a “sleep sack” are you referring to a sleeping bag or swaddle suit? ie are baby’s arms in or out of the product?
If arms are in (swaddle suit) then it is a swaddle.

Have a look at our general information on bedding & clothing for sleep. It is not possible to provide specific information for each baby when it comes to type or amount of clothing/bedding.
These should provide you with some tips when deciding on bedding/clothing for newborn.
https://rednose.org.au/article/how-much-bedding-does-baby-need
https://rednose.org.au/article/how-to-dress-baby-for-sleep

https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep