Either do a block test and see what colour the dye goes or pop round to your local MOT place and hover the CO tester by the header tank. If theres a reading of co's its your h/gasket. Also check your turbo for wear and see how much oil is in the boost pipes.

Does it just to do it on satrt up? Possibly valve stem seals.

You'll need to be a bit more specific m8 for an over the net diagnosis, as to when, etc, etc, etc .....