Looks like the turbo solenoid has always been overboosting. I did post about the turbo whistle being odd when I got it.
New hoses and vacuum pipes and sealed up top of engine have pushed it over the edge.

So no fault with work done, likely revealed an old problem.
Hope a new solenoid doesn't impact performance too much !!

Fitted new turbo solenoid. No more power reductions and no fault codes for boost problems, however, the live data states that the boost is still hitting 248 Kpa inlet manifold pressure on occasions.

None of my other TD4's get over 220 Kpa and it does seem that 248 is the max so not sure what is happening here. Maybe it has been 'chipped' in the past?

For anyone wanting to check the turbo solenoid easily, if you fit a large syringe to the 'Vac in' connection, then pull syringe back, it should offer full resistance. If it doesn't then the solenoid is stuck open as my old one was. I guess then, the engine vacuum rather than the ECU controls the vanes, or as was happening to mine, maybe both influence the turbo servo causing it to max out when not expected.

Will use it for a bit and report back.

The good news is, it isn't any slower !!