I said before it went away for the repair "If something goes wrong, anything, within the first month after I get it back, it's going!" and I'm sort of holding myself to that
The mechanic assures me it was boosting happily to a stable 1.2 bar when it left the garage, so it sounds like something, somewhere has popped off. The boost controller, despite being plumbed in correctly, does not yield any changes in boost pressure no matter what the settings are. At least I've got something to go on now! (Only just got the text back)
That said, the first thing I checked was all the vac hoses...
Done that, same results!