My DS was a tummy sleeper.....I would settle him in that position. He was also a slacker though and never moved apart from turning his head from one side to the other!

I had him in a grobag (I think it was 0.5tog for Summer) which stopped me worrying about him kicking off covers. It would also stop the legs through the bars bit. I also bought an Angelcare Monitor....where you put the sensor under the mattress and it constantly picks up movement (breathing) which put my mind at ease at night time.

I was a tummy sleeper too and still am, it's such a comfy posi!!! (regardless of being 17 weeks pregnant!)