My baby is 5.5 months old and has been able to roll for the last 2 or so weeks. She can go from back to front really well and is still mastering front to back. We have moved her into her cot from her bassinet. I always put her to sleep on her back and then usually half an hour later she will try to roll on to her tummy. I think because she is asleep at times she can’t quite make it all the way over she, will wake herself up and cry out for me (other times she manages it fine). Now that she can roll can I place her on her tummy to go to sleep or can I help her roll on to her stomach when I notice that she might be trying to roll over but having trouble? Our cot is also quite narrow so often once she has rolled I have to move her away from the sides of the cot anyway.

Red Nose recommends that you always place baby on the back from birth, even when they are rolling.
Red Nose has the following general information now that she is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage 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

Does your cot meet the Mandatory Australian Standards?