As yet i haven't had time to rummage around in the injun bay, but i may have a possible lead on it.
I 'think' it may have something to do with one of the turbo to manifold bolts having sheared off, there is a tiny gap between the area of the turbo and where it touches the manifold. When reved, i get exhaust coming out of the gap, not a major amount but enough for it to be seen easily.
This doesn't explain the white smoke on high revs, but could it explain the lack of top end revs?
I will get my mitts grubby in the next few days for a good look at various other areas