Thanks guys. I did swap the ignition control units - the car would run as rough as before and I was not able to tell which cylinders were affected after swapping. Error codes are now no longer appearing after clearing them but the car still runs rough and dies. If I leave only one unit connected at a time, car will only start with one of them so my idea was that it might be actually it.
Having also posted on Polish A8 forum,
http://www.a8team.pl/, guys in there are saying this is a fuel pump problem and it should be replaced. Could that be the case?
Will check the measuring blocks later on today.
Cheers.
Tom