Thanks for updating your progress on this aerosam, I've been having the same problems with mine, not quite as bad as yours (its also a 2003 JTS 128,000 miles...). Very lumpy at idle. Fault code says lambda failures all round and misfiring on cylinder 3.
Had the problem for 6 months now, been dragging my heels with it a bit, hoping it would go away!
Well actually so far I've checked the connections at the back of the engine bay, cleaned them up, same for the lambda connections at the front. (No change) Stuck some injector cleaner in the tank (no change) also sprayed a can of direct injection inlet valve cleaner into the air intake (noticed a slight improvement). Also changed the spark plugs and swapped around pre cat lambda sensors with 2 other used ones i got hold of...And reset the ECU... But I'm still getting same messages, lumpy at idle, engine light on, misfiring on 3 etc, and i dont think i'm getting the full 165 BHP and also getting very crappy economy (around 25mpg at best)
Guess the next stage is to think about changing out injector 3 as its always this one on the error message.
Out of interest what did they charge you for this - how many hours labour was it...?