Car was running fine up to when it went into local Audi dealer for a routine service + recoding some spare transponder keys. Dealer reported that faults with engine management system were showing which would require more diagnostic time to trace. But car was virtually undrivable upon return from dealer and rapidly became almost totally undrivable - erratic running, almost no engine power intermittently etc.
So car recovered to dealers who after much diagnostics have advised that it needs a replacement ECU - needless to say at a very steep estimated price, but I now really have no choice but to get it fitted so that the car will run properly again.
Suppose the fault and the servicing could just have coincided and suppose nothing is provable either way, but I'm just posting this in case this is a known / common fault on this model or if anyone can offer any further observations / explanations.
David Cumbria, UK