Thanks Bindy and AJP that does help! Will try lying him with a towel to encourage sleeping to his other side plus we do know he may need to see a physio if the problem doesn't resolve itself.

Our main conern at the moment is that he does actually move his head to both sides quite easily, and lying him at the other end of his cot does now see him lie with his head to the right (rather than the left which is causing the flattening), but that he is really unsettled and scoots himself around his cot and thus he isn't getting any sleep!