Harty i got exactly the same problem. It only happened since fitting new turbo last week. Does it sound tappety as well? I have been told a few things
1. timing
2. air leak
3. plus
4. cam seal
Let me know if you solve it. The guy who did me engine th eother week (low comp) he is coming tomorrow to have a look so if he solves my prob i will let you know dude