Have a A500, but Bizarrely I have exactly the same fault code and same systems, after just having had the exhaust manifold gaskets replaced due to a small leak on start up.
Same symptoms, half boost, but sometimes it does boost well, but immediately drops.
To date I have changed the turbo boost solenoid and all pipe work, to no avail.
Next step is to change the boost sensors and check all the pipe work etc.
Could be a variety of things, but have to work through them all.

Seems strange however, everything was working so well before the work, might be coincidence but! Strange both vehicles have had similar things done to them.