Hi Tony,
Thanks for your reply. Yeah I agree the dull srews which are missing seem to be missing for a while.
Though, in the photo with black oily residue, there does seem to be a threaded hole which has fresh oil visible, also there seems to be evidence of oil almost 'spraying around the head manifold / or turbo.
I do not quite think it is, although I found a diagram of the engine head manifold, and it looks like 4 bolts hold, and seal various gaskets, and I would think create a seal, and therefore affects pressure. Loss of seal, would equate to loss of pressure, and then possible power from turbo, if this is a major screw.