Hi I've started to put my 6.5month old to sleep in her cot during the day. Bassinet with me at night. I have noticed that because the cot is bigger she is free to roll onto her belly and finds this position comfortable to sleep. Do I need to roll her over? She confidently rolls from front to back and back to front.

What you describe is a normal phase of development. Most babies will be rolling by 4-6 months of age.
It is important now, that your baby is in her cot for all sleeps - she needs that space to roll safely.
Also ensure that mattress is firm & well-fitting & that there is nothing loose, soft or padded in the cot.
Wrapping should be ceased, if you haven’t already.
Always place her on the back for sleep.

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