Peugeot 407 won't start

I have a Peugeot 407, 1.6hdi that won't start. It will start when rolled but won't start with the turn of the key. It was happening a few weeks ago and I assumed it was the starter so replaced the starter. It worked perfectly for 2 weeks but now it's happening again. All lights on dash work but it just won't start. Any ideas??