Hi, my 5 month old is now rolling onto her tummy in her cot, but hasn’t yet learned to roll onto her back. Is it safe to leave her on her tummy? Or should I be going in throughout the night to roll her onto her back again?

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

Have you now ceased swaddling/wrapping her?
It is important now that she is in her cot.
Babies need arms free & the space of a cot to roll well & safely.
Ensure mattress is firm & flat & nothing soft or loose in her cot.
Continue to always place your baby on back for sleep.

How does she go with tummy time when awake?
Increasing tummy time when awake is important to build upper body strength to roll well (both ways)

As she is not rolling both ways we would encourage you to gently roll her back when you see her on her tummy, but you are not expected to sit up all night watching her.
What happens when you roll her back?