My nearly 5 month old has started sleeping on her side in the cot. Rolling from back to front began early at about 3 months, however she is unable to roll front to back yet. I have used a sleep suit since she was around 1 month and transitioned to the appropriate arms free one once she started showing signs of rolling. I have also tried a tight top sheet to keep her from rolling. This worked for a night, however she managed to still get to her side the following night. Her cot is next to my bed I am frequently waking to check on her, trying to roll her back to her back if I find her on her side. She usually resists it and often either wakes or fights being put on her back. I give her lots of tummy time during the day and am trying to encourage her to learn the roll back to back skill. How can I keep her on her back the whole night to improve both our sleeps?!

Most babies will be rolling over by 5-6 months. This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy.
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.

Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Preferable to use a sleeping bag now that has appropriate size neck & arm holes, which I see you are doing.

Red Nose has the following general information now that your baby is starting to roll.

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.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Do not use any devices designed to keep baby in a particular sleep position.
Products that restrict the movement of a baby or a baby’s head should not be used.
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position