I've had exactly the same problem with DD now 7m. A grobag definitely helped. I've also put a rolled up towel under the sheet at the top of the cot which helps to stop her wriggling upwards and getting her head stuck between the top bars. Good luck!
dd2 sleeps on her tummy at 7mo and spends most nights with her arm hanging out through the bars - doesn't seem to wake her up though. She is in a Grobag and stiill sometimes manahes to get her legs stuck!
you buy one of these if it gets really bad - but I suppose have to balance against ability to use it to escape!