Hi, my 6 month old has just learned to roll back to belly. He has not yet learned to roll belly to back (though he has done it on the odd occasion, it appeared accidental). He is strong in tummy time, can do 15-20 minutes at a time. He has been sleeping with arms out for three months. He sleeps very well for 13 hours overnight, he is in his own room. Do I need to be waking myself up to check whether he has rolled into his belly during the night? Is there a worry that because he doesn't know how to roll tummy to back he may get stuck on his tummy and this could be dangerous?

Most babies will be rolling over by 4- 5 months.
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

Always, continue to place him on his back for sleep.
Glad to hear you have his “arms out ” now.
Has he also moved from basinet to a cot?
Babies need arms free & the space of a cot to roll safely.

Parents do not need to wake up (or stay up) to check on their baby overnight, but if you are awake & up, then have a check on him.
Have you tried to roll him back when you see him on his tummy? What does he do?
He is most likely rolling back intentionally when he does roll.
It is important to know that he is capable of rolling both ways.

Give him as much tummy time when awake, as is possible.
Sometimes it is preferable to increase the number of times per day & not only amount of time.
This will also help to increase upper body strength & help him to roll well.