I have a 13 month old. As it is getting colder, what is safe to put in the cot to keep her warm? Are a sheet and blanket safe or sleep suit?

Red Nose has the following information about bedding:
Dress your baby as you would dress yourself for the temperature of the room – comfortably warm, not too hot, not too cold.
In warmer weather, you can dress baby in a sleeveless and legless onesies, or just a nappy and singlet. For cooler weather, a full suit will keep baby warm.

Make up baby’s bed so baby sleeps at the bottom of the cot and the blankets can only reach as far as baby’s chest, ensuring baby cannot move down during sleep and get his/her head covered by bedding.
Consider using a safe baby sleeping bag (one with fitted neck, armholes or sleeves and no hood).
Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
https://rednose.org.au/article/bedding-amount-recommended-for-safe-sleep

Does your baby stay covered when using a blanket?
if not, then instead of using a blanket, add an extra layer of clothing to baby or a sleeping bag of increased TOG rating suitable for colder weather.