Hi, I always thought that polyester should be avoided for baby’s bedding due to risk of overheating. I have recently noticed that some of the popular branded sleeping bags have polyester filling in the 2.5tog bags (and cotton inner/outer linings) . Are these bags safe to use or should they also be avoided?

Red Nose recommendation for a safe sleeping bag is one that is the right size for the baby with appropriate size neck & arm hole - ie it fits properly -  & has no hood.
Red Nose does not recommend any specific fabric to use (or not use)
If the fabric is warmer than another, then you would use less covers/bedding for baby to reduce any risk of overheating.

A safe baby sleeping bag is constructed in such a way that the baby cannot slip inside the bag and become completely covered.

The sleeping bag should be the correct size for the baby with a fitted neck, armholes (or sleeves) and no hood.

When using a sleeping bag, ensure that baby is dressed according to the room temperature.
https://rednose.org.au/article/what-is-a-safe-sleeping-bag