Martin
I have a 2005 2.5 V6 petrol with 62,000 miles and have been through the very same experience as you describe. I am not categorically stating that my problem is the solve of your issue, just posing it as "food for thought".
Have replaced all six ignition plugs and ignition coils, along with new upstream and downstream Lambda`s, had smoke test, new exhaust manifold gaskets (6), all OEM specification etc. etc. After the misfire, which was only on one cylinder (No3 in Bank 1 ) all went well for a couple of weeks. Then the code P0420 started appearing and would reappear after being cancelled. This indicated a Catalytic Converter problem on Bank 1. Eventually had the car up on a lift and found that the pipe from CAT on Bank 1 was, if tapped heavily with a fist, making a rattling sound as though as though full of bits of rubbish. A very experienced technician that was with me immediately knew that this was a bad CAT that was causing the Yellow Engine Warning Light and Code P0420. He had seen it many times before.
It is apparently known that a misfiring engine can eventually "kill" a CAT, which my thinking is that it has happened to mine.
None of the usual signs of CAT failure (high fuel usage, engine not running smooth, smell from the exhaust of rotten eggs etc etc) were present.
I have now purchased a new Catalytic Converter (along with necessary fitting kit) and will hopefully this coming weekend be fitting the new CAT.
Will report back on this matter once CAT is fitted and run for the 18 - 20 miles that it usually takes to make the P0420 code appear.
Hope this may help in some fashion or form and good luck with resolving your issue.
All the Best and Regards, John