Thanks for your replies guys Sorry I never popped back in, we've been fighting various battles for the last couple of weeks (mostly sleep related, sigh).

We're in the middle of moving at the moment, so for nights we have our mattress on the floor and bubs on her little cot mattress next to me - she loves it lol. So for now the hammock is only being used during the day and it's given me a good chance to watch her in there. She can roll when she's on the floor, but has no hope of rolling in the hammock. When she wakes she stretches out, and just waits for me to come get her lol. So I'm happy that she can't roll into the mattress or anything, and I think because she's not so small and she's very strong she can get herself in a comfy position without her chin down on her chest.

My hammock sounds a little more like the one your midwives described Nae, the mattress isn't firm, it's kind of like a quilt but not (as if that makes any sense!!). Anyway it's not really flat, it curls up around her more. Hence not being able to roll in it. I've found the air circulation to be pretty good, but now that it's getting warmer here I don't think I'd like her in it overnight anyway.

Thanks for all your advice guys, I guess it is like anything else baby related - know the risks and weigh it up!