As colder nights approach advice on bedding with swaddling

[dibley30]

My ds is 9 weeks and swaddled in a miracle blanket (mothercare) - I'm not proposing to get rid of it at the moment and move to the grobag as it has made a massive difference to how well he sleeps during the night. We started with a grobag after he reached the 7 and a half pound mark but then went back a step to swaddle after various settling at night problems . As the colder nights approach I'm wondering what else to put on the bed - the grobag obviously gives you some guidance but I can't find similar info with miracle blanket. I don't want him waking up cold. The nursery is around 20 - 21 degrees at the moment but it gets reeeeealllly cold in that room in winter. At the moment I put two small rolled up towels either side of him with a cotton sheet over the top of him and the towels tucked into the mattress either side. The miracle blanket is thin stretchy cotton and he has a cotton short sleeve vest and then long sleeve cotton babygro on too.
Any advice welcomed !
Thanks

[MuddlingThru]

I would go by the grobag guidelines but consider the miracle blanket as one of the clothing layers, eg at 18c it says long sleeve vest & PJ top plus 2.5 tog, so I would probably do long sleeve vest, miracle blanket plus 2.5tog.