I put my 11m old in a sleeping bag & onto her back to sleep. She rolls onto her tummy, wriggles to the top of the cot and Loves to sleep with her face in the mattress. If I turn her on her back, she moves right back to this position. What do I do to keep her safe? When she feels cold, how can I safely use a blanket if she’s moving to the top of the cot?

What your baby is doing is normal.
Most babies will be rolling by 6 mths of age - some earlier, some later.
This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur. 

It is important to ensure the following:
Always place baby on their back to sleep.
Ensure cot mattress is firm, well-fitting & flat.
The incidence of SUDI at 11 months of age is extremely low.

Red Nose has the following information:

As babies grow and develop they become very active and learn to roll around in the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own natural sleeping position. By this stage it is not necessary to wake during the night to turn baby over to the back position.

Do not use any devices designed to keep baby in a particular sleep position.
These can be dangerous and they are not recommended.

https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
As she is moving around in her cot, there is little use for a blanket.
Dress baby in her sleeping bag, & add other layer(s) of clothing under the sleeping bag.