You may well be already doing it, but when my bub was doing this, I slept him in a sleeping bag and very tightly tucked sheet and light blanket over the top... it will usually keep him in place! Some linen is better than others, and I've even tucked some in long ways IYKWIM... i.e. instead of the same orientation to the mattress, the long side is going across, with all ends very securly tucked (given they they're still not THAT tall in the cot at this age.

So saying, I think this is actually against SIDS recommendations - I think you're supposed to use a sleeping back without extra linen, but it does work for us. GL!