Lying down on back

If you are propped up and not completely flat, I believe that is ok. It's if you are totally flat, then full weight of the uterus is on top of an important blood vessel.

using a couple pillows but thank you :)

I think they changed the nhs guidance to say ok up to 28 weeks? It used to be earlier..

https://www.nhs.uk/pregnancy/related-conditions/common-symptoms/tiredness/#:~:text=Bump%2Dfriendly%20sleep%20positions,and%20oxygen%20to%20the%20baby.

Then from 28/30 weeks as well as risks sleeping on back it’s also about getting baby in a good position. E.g not slouching back on sofa to watch tv - trying to sit on birth ball with hips above knees instead. Or side lying if need a rest