My son is 12months old and has always slept in a sleep swaddle/sleep suit. Last winter he was swaddled and I had blankets over him and tucked under his bassinet mattress to keep him warmer as per SIDS guidelines, however now he moves freely around his cot I am apprehensive to place a blanket over him for more warmth, but think it’s something he needs some nights. Would it be okay to do so now he’s 12m+?

Red Nose does not have any specific age recommendations for the use of blankets & has the following information about the use of blankets in baby’s less than 12 months of age.
It is important that baby’s head & face remain uncovered.

If blankets are being used instead of a sleeping bag, it is recommended to place baby with his or her feet at the bottom of the cot, using layers of lightweight blankets that can be added or removed easily according to the room temperature. Tuck blankets in firmly so they cannot become loose and cover baby’s head during sleep.
https://rednose.org.au/article/how-much-bedding-does-baby-need

Many parents report that once baby becomes mobile &/or is rolling, they do not tend to stay underneath blankets.

Red Nose suggests that it is easier to keep baby warm by using a sleeping bag & add more layers of clothing to baby when weather is very cold.
Red Nose recommends that you avoid using any soft, padded, loose bedding until a child is in a bed. Some babies have been known to use these items to help them climb out of the cot.
https://rednose.org.au/article/at-what-age-can-i-introduce-cot-bumpers-and-pillows