@richardindorset I am unsure if I am having the same issue as you diagnosed. I also get the p0299 underboot, however I don't have access to a garage right now, but I could log some stats. It seems to me that my VGTDC stays low after it logs the p0299 underboost DTC. Did you notice anything like that?

My car is a 08' 90hp focus mk2 1.6tdci. Normally when driving a turbo diesel, you can kind of feel when the turbo kicks in at around 2500rpm ish (give or take the model), however I newer ever recollect having had that feel in this 90hp focus. It has the same flat sluggish acceleration all the way up to 120km/t (75mph)

The PIDs that is shown in the graph


Acceleration onto the freeway.
Notice how the VGTDC (orange) newer seems to rise after the DTC (P0299 underboost) is logged, even though the speeder (APP,% cyan) is floored before and after. By coincidence the DTC is logged during the gear change from 2nd to 3rd,