After 8 months of sleepless nights (on average waking up six times) DS2 only woke once last night and then went straight back to sleep. I'd put a pillow in his cot.
I put it in between the mattress and the fitted sheet. Plus he has a breathing monitor (although I'm aware of course that this will not prevent a situation where he stops breathing, only alert me to it).
He is rolling over and crawling now. He sits up on his own in the cot despite being in a sleeping bag.
Am I doing the wrong thing? It seemed to do the trick.