I have a 5 day old newborn. As soon as you put him down for nappies, in the bassinet, in the cot, wherever, he immediately rolls onto his side and it's not the same direction each time. This is my second baby so I know about safe sleep, we are using the same bassinet and cot mattress as we did for my first so I know it's not those and just something he does. We are using the love to dream swaddle bags and I've tried tucking him in as tightly as possible but still does it and no amount of readjusting stops. What do I do?

Most babies do not roll to tummy intentionally before 3 months; however, some babies do roll or wriggle, especially to the side, at a very early age. This is not unusual.

Some babies do not respond well to wrapping/swaddling & prefer to have their arms free.         
Some babies like to touch their face or suck fingers when settling.
You could try settling him without any swaddle, or wrap with his arms out. (as for 3 mth old)

https://rednose.org.au/article/wrapping-babies
Red Nose recommends using a light cotton or muslin wrap rather than swaddle suit like the one you are using.
Did you try a muslin wrap before using the swaddle suit/bag?

He may be more settled in a cot for all sleeps rather than a bassinet.

Have you commenced tummy time yet when awake?
https://rednose.org.au/article/tummy-time-is-good-for-baby-because
Always place baby on the back for sleep.

You are not expected to wake during the night to keep checking on him.
However, there is nothing wrong with rolling him back if you see him on his side.

Red Nose does not recommend the use of sleep positioners.
Red Nose does not recommend positional products such as anti-roll devices and items that fasten a baby into a sleeping 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