Hi, my 14 week old has started to roll onto his side at times when he is asleep. We have stopped swaddling him and have his arms out, but he is still in his basinett, it's a large co sleeper basinett but I am wondering if I have to stop using this and put him in his cot? My concern is that his cot is in his own room and not in ours and I don't think it will fit in our room. I know it's recommended they sleep in our room for the first 6 months. So what do I do?

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

What happens if you try to gently roll him back when you see him on his side?
You are correct, it is recommended that babies are placed in a cot once they are rolling.
Babies need the space of a cot & arms free to roll well.
Some parents use a portacot to continue room sharing for a few months, although preference is for a standard cot in your room if it will fit.
If he is in his own room, check on him when you wake during the night.
A safe environment is important.

How does he go with tummy time when awake?
Increasing tummy time when awake is important to build upper body strength to roll well (both ways)
Is he rolling during tummy time, holding his head well & reaching for toys?

Continue to place him on his back for sleep.
Parents are not expected to sit up all night observing baby.