Jaguar XJ8 - Rough Idling

My workshop checked my car thoroughly and could not find what is
causing the rough idling in my car. They finally suggested that they
change the ECU unit( I am not too sure if that is what its called) This
unit cost quite a bomb new. My question is can this ECU unit be
repaired? If not can a good used one be used instead. According to my
workshop it cannot be done as the computer units are programmed and
must be made to suit the car. Being such expensive equipment i am sure
Jaguar should provide some form of warranty. I would appreciate some
advice from anyone who have experienced this. Thanks. Syed