Unfortunately if I had done the job the fault lies with you and that's because you prepared it all. The ceiling hasn't cracked because it's to thin it looks an alright job from the pics and they taped the edges so I guess they would tape the joints. You say you battened across the original joist what you have also done is batten along the edges of the boards and put plenty of screws I doubt there was any room for movement once it dried, also have you got any pics of the battening? I would on a guess say you used roofing battens and if these were wet this will also cause it to crack. Your only chance to blame the plasterers and you are looking to blame is if they didn't tape the joints, if they did its on your toes.