I had this problem with the S2 Roadster and still have it with the Aero Plus 8.
Always the N/S downstream lambda sensor slow to respond.
After talking to may people and trying different fuels I started keeping a diary, noting when it happened the weather, driving conditions and style. I've done this for over 30,000 miles.
Conclusions: it happens when the cat gets cold and isn't working optimally. This seems to be caused by the weather and by how you drive: if I keep the revs above 30% and vary the throttle opening frequently it doesn't happen, so enjoying the car on a B road it is fine.
If I have been pootling along in top at 50mph in traffic on an A road, then suddenly change down and accelerate hard to overtake it will most probably happen and if it is below 10C then it will certainly happen
Choice of fuel doesn't seem to make much, if any, difference.
Short of insulating the cats and exhausts and fitting spacers to the cats I don't see a solution.
It is easier to clear the light when it happens.
Bottom line: "they all do that sir"
More seriously, it isn't a fault, it is an observation and it is caused by putting a large and powerful engine in a small light car, so it isn't working very hard. So drive it like you stole it and all will be well...
