This sounds very similar to the problem that keeps occurring on the 3.2.
If the setup is similar, the post cat sensor is after a small "first stage" cat but before the main cat. For some reason, this sensor appears to be troublesome but, even if the emissions after the first stage cat are problematic, the main cat clears the emissions up so they are within the MOT limits.
I have had this problem crop up a few times on my 3.2, as have a number of members, but AFAIK it's not common on a 2.2.
The theory is that the first stage cat is a little small to handle the flow of the 3.2 engine and hence doesn't clear up the emissions adequately. Once this has happened a few times the light comes on, and it will clear if the emissions remain OK for a certain number of engine start/stop cycles.
The problem is unlikely to be a faulty lambda sensor. Some members have reported that a dose of "cataclean" stops the fault recurring and I would be inclined to try this as a next step, followed by clearing the codes and seeing if the light stays off.
If this doesn't work, and bearing in mind that it's not a common problem on the 2.2, cleaning (in an ultrasonic bath) or replacing the post cat sensor might be worth a try.
As with all such problems, make sure the inlet, and exhast system upstream of the sensor have no leaks.
Kevin