I've had a worsening flat spot on the TD4 and DTC P0045 for two months (with occasional Reduced Engine Performance errors probably due to low fuel pressure) but the flat spot has disappeared after the technician freed up a sticky turbo vane actuator. The engine had plenty of power but now it is extremely smooth all the way up from 1000 rpm. I guess I was convinced that "turbo lag" was an inherent design "feature" but it seems this is not true.

The generic P0045 points to an open circuit but it never turns on the MIL. I can't check the subcode but it seems more likely to be high current in the actuator driver circuit since freeing up the actuator has reduced the load on the motor so the over-current detection is not triggered. The other possibility is the ECU calling for a certain vane position but the position sensor was returning a lower reading.

Anyway, it's good to know there was a simple cure and I hope this helps anyone else with a flat spot.