The clue to all this is that on the later Omega's (Don't know much about the earlier cars setup) there is an Emissions lamp and a seperate Engine Management Lamp. It's the emission lamp that the 3.2's are triggering, not the Engine Management.
The Emissions lamp is the same icon as the Engine management lamp on most other Vauxhalls - the picture of the engine. The Engine management light is the picture of the car with the spanner through it, this can and does confuse most garages, even VX themselves.
If the emissions light flashes you are supposed to only lightly use the throttle because something (contamination probably) is possibly going to damage the CATs. If the light is on permanently then you can drive normally but the emissions system (Ie the CATs and the lamda sensors) thinks the emissions are slightly to high over a period of time, ie. it's reached it's threshold or limits over a period of time.
In my case I had the lamp on continuosly, I had the Emissions checked at an MOT Station and the CAT's are working well within the MOT requirements, infact they are virtually as good as new, all four Lambda sensors were working as they should, the only test that wasn't run was running live data over a period of time to gain a better picture.
I have a theory that the sensors on the 3.2, being a different design to the other V6's are slightly unproven over a long term, I also think the varying prices of petrol is a good indication of different qualities of petrol. The emissions light has only come on again since I started using V power, If I stick to the same brand for a few months I don't think the lamp will return, if I go and put a tank of Tesco cheap 'n' cheerful in I bet it comes back on.
If the engine side of the sytem is at fault (on my car at least) then it's not a fault that the Engine Management is recording, so really all you can do is swap a part every few months to try and solve the problem.....