When I had J (he was a wee prem) we lived on Orkney and had a wood fired heating system which often blew out in strong winds, which meant it could get down to 13C at night. So I utterly panicked about keeping him warm at night. Now we're in a drafty Victorian and get a huge temperature range in the bedrooms over the night across the year (from 27C this summer to 14C last winter).
But general rule of thumb that we have here is 2.5 tog sleeping bag with long sleeved vest and sleepsuit (the ones with arms and legs) or the same outfit with a fluffy swaddle and a blanket (when he was wee) for temperatures between 17-13C (and I'd pop a blanket on top of his sleeping bag for anything below 15 but J hates the cold). 2.5 tog and long sleeved vest for 19-17C. 2 or 1.5 tog with long sleeved vest for 19-23C. Short sleeved vests and 1 tog sleeping bag for anything over 23C. It really does depend on your kid. Also note how if you have to stay at night the hospital will totally disregard SIDS advice and wrap your baby up super warm to get them to sleep.
If they are in a bedside cot they get their breathing cues from you - having them close (especially in the same room) is much more important for preventing SIDS than worrying about a precise number of layers.
The key is that if you pop your finger under their clothes onto their chests - they should feel neither hot nor cold to the touch (i.e. they should be at body temperature). Get a gro-egg thermometer now so you know how the room behaves. For us, I know the room gets hotter and hotter until about 10 then cools off, with the coldest point of the night (which can wake him up) being about 4am. So on a cool night I'd often pop a blanket on top of him as we go up to bed. Obviously don't put a loose blanket on a newborn - as they can get smothered by them. If using a blanket you have to pin it down tightly into the cot sides and baby should be feet to foot (i.e. feet to the foot of the cot) so that they can't wriggle down under a blanket.