My baby is 5 months old and has begun rolling onto her stomach for every sleep after being placed on her back. She has a habit of resting her face on top of her hands or pressed straight into the mattress. I turn her head to the side when I can however this has become a stress factor overnight when my partner and I are sleeping. I have researched the dangers of rebreathing and this is my main concern with her positioning. She has a cot and firm mattress that meet safety standards. Nothing else is in her cot and both arms have been removed from her sleep suit, she is also able to roll each way. She seems to prefer to have her face in this position so will just readjust back into it when we move her. Should I be concerned about re breathing?

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over 

It is important you have moved her from bassinet to cot, & ceased swaddling/wrapping.
Babies need the space of a cot & arms free to roll well.

Some babies do tend to face plant for a time. - mattress firmness is important - most babies who face plant will lift head & change position or move position. A well-ventilated room (as well as a firm mattress & baby lifting head/moving) also help to reduce the risk of rebreathing.

You could gently turn her face to the side if you see her face planted.
Parents are not expected to sit up all night watching their baby.
As she is able to roll both ways, still place her on her back, but let her find her own position of comfort.  (see linked article above)