Had a look at the vacuum pump on mine today. That spigot seems to move in and out on mine too, but I didn't hear any sounds of air leaking when I moved it.

I am wondering if the leaking of air was from the other split I identified today at the other end of that pipe, and therefore it wasn't actually at the vacuum pump end leaking....

I popped the hose off, and tried to tap home that spigot thing with a piece of wood and a leatherish mallet thing (so as not to break anything), and it did not seem too want to seat itself any better. Reattached the hose with jubilee clip rather than trying to re-use the once-use clip which was on there.

Well i recently had the same issue with overboost, a dirty MAF can contribute to an invalid air reading and hence cause overboost. Give it and your N75 a good clean and double check all of your vacuum cables. Has your air filter been changed recently? Try an N75 test on your vagcom to rule out a dodgy N75.

I have attached a guide to help you sourced from tdiclub.com forum

LimpModeDiagnosisGuide.pdf

most of that info is for a wastegated turbo found on early 90bhp cars (and us cars) the airflow meter can thro up spurious faults but not the overboost code that is only triggered by too much boost.

it can trigger a differance in expected pressures code (can't remember what it is tho) as the ecu may see lots of airflow (falsely) and no corrasponding rise in boost.