Hiya

Ive had this a few times with vauxhalls, had one last week with same engine code and year actually. Sounds like you've checked the obvious but i'll run through what i check anyway, not in any order. Hope it helps.

Check the plugs and coil.

Replace the EGR valve, they have a habbit of staying open and its often a waste of time cleaning them, and only fit genuine as ive has too many comebacks from anything other than genuine. And yes there is a modification to do away with this valve.

Check the injectors, swap them around and clear the codes and see if the misfire moves, but ive never had an injector give grief on vauxhalls.

Check/replace the lambda sensor, but thats normally a short term cure. And i cant see the sensor giving you this problem.

What ive often found is the engine is burning oil, causing poor spark/random misfires and the ECU cuts the injector off on the worst cylinder. Test it by running the engine from cold with the scanner connected and set up to view the lambda signal as a graph, and at the same time have a gas tester up it. Hold the RPM at about 2000 and watch the CO% and O2 readings on the gas against the lambda on the scanner, just before the light comes on and the injector is shut off the CO% will go 1% 2% etc, the O2 will go 6 7 8 etc, and the lambda sensor on the scanner will indicate rich for a second then go constant lean. If you watch the lambda sensor graph carefully you will see it building up to the misfire as occasionally the sensor will only peak to about 500-600mv. Take the plugs out and have a look down the bores with a light, you should see them glisten back at you.

I wouldnt get too down about that customer having an "expert" fit a lambda sensor, i think they will be back with the vehicle with the fault still there.