My 6 month old has started to sleep face down in her cot with her hands under her face. I am worried if this will increase her risk of SIDS She is put down on her back in an arms free sleep bag but rolls. I try to move her face but worried if I miss seeing her in this position

Ensure that the cot mattress fits well & is firm & flat & there is nothing soft or loose in her cot, especially as she is face planting at times.
You can try to turn her face if you see her with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
Increasing tummy time when awake is recommended to help build upper body strength so she can roll easily & well.
Always continue to place baby on back to sleep.

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over