My 11 month old has recently starting sleeping face down with forehead against mattress. When I roll her over to her back she rolls straight back to the face down position. She sleeps in her own cot on firm mattress and is rolling/crawling/walking with assistance. Is it safe to let her sleep that way or should I keep rolling her on her back?

Has she been rolling for a while but has just started to face plant?
Does she have her hands under her forehead when face planted?
I appreciate that this can be alarming for parents to find their baby face down in sleep environment.
What happens when you turn her face to the side when you see her face down?

It is very normal for babies to roll from about 4-5mths & many will prefer to sleep on their tummy or side.
As babies grow and develop they become very active and learn to roll around in the cot.

Once baby can roll confidently from front to back, and from back to front, continue to settle them on their back, but allow them to find their own natural sleeping position. Once baby is able to roll in both directions, it is not necessary to wake during the night to turn baby over to the back position
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Things that are important to reduce the risk in relation to rolling & face planting.
Always place baby on back for sleep.
Cease swaddling or wrapping - baby need arms free.
Ensure baby is sleeping in a cot
Ensure mattress is firm, well-fitting & flat.
Nothing loose or soft in cot.

Sleep positioners are not recommended - https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position