We don't have reliable heating in our home so we don't use it overnight, recently the temperature has been getting quite low and I think bubs is getting cold but I'm not sure how many layers is too many! The room temp has been as low as 15 degrees. Any tips on safe dressing overnight?

If your baby is settling ok, sleeping ok (for age) & not waking at unusual times during sleep (especially in colder months) your baby is probably warm enough.

It can be difficult to work out what may be the most appropriate clothing/bedding for baby when sleeping.
Red Nose recommends dressing baby according to the room temperature, rather than trying to modify the room temperature with heating or cooling.
It is not possible to give you specific guidance for dressing your baby, as every baby is different.

Red Nose has the following general information.
“Dress baby and use layers as you would dress or use layers yourself: to be comfortable, neither too hot nor too cold.
“Dress baby for sleep and add/remove lightweight blankets to ensure baby’s back or tummy feels comfortably warm to the touch.
Remove hats, bonnets, beanies and hooded clothing from baby’s head as soon as baby is indoors.”  -
Always sleep baby with head & face uncovered.
Always place baby on back for sleep
“A good way to check baby’s temperature is to feel baby’s back or tummy (don’t worry if baby’s hands and feet feel cool - this is normal)...................
Consider how many layers that you as the baby’s carer are wearing comfortably.”
https://rednose.org.au/article/how-much-bedding-does-baby-need
“As long as baby is put down to sleep on their back, and that baby is dressed appropriately for the room temperature – not overdressed or under dressed – with their head and face uncovered, you can feel reassured that you are protecting baby from overheating.
We don’t believe that it’s necessary to use a room temperature monitor, or to leave the heating or cooling on all night, as long as baby is dressed appropriately for the temperature of the room – not too hot, not too cold.”
https://rednose.org.au/article/how-to-dress-baby-for-sleep