She is nearly 5 months old, and ever since she became proficient at rolling from her back to her tummy a few weeks ago she has rolled herself onto her tummy to sleep.
The issue is that at night time she sleeps in a 'next to me' cot attached to the side of my bed. If she rolls herself over once I have laid her to sleep on her back she ends up lying over the join between her mattress and mine. I've stuffed that gap as much as possible, but is still a small gap. If I lie her straight onto tummy I can put her down in the middle of her own mattress.