Right, so it appears the answer to my original question here is .... 4!!
I got the car up on the ramps this afternoon and slid myself under to see where I could see the sensor, found 2 around the cat, 1 just before and 1 actually in the cat. Here's a photo looking up at the cat:
So, having only ordered 1 new O2 sensor I did a bit more research and discovered a) on a V5 there is allegedly 4 sensors (2 on the downpipe and 2 around the cat, and b) the S2 in the fault code means the second one of the cat, S1 being the first one.
One important note, they are both in an extremely difficult location to get to with any normal tools!! Now I know why ECS sell a couple of special tools for this. Which of course I didn't buy.
As it happens, I ran the VAG-COM diag again and there are no faults being shown at all. So as this specific error appears to be very common, as revealed by a web search earlier, I have decided to leave it for now. I don't believe the fault code means it has failed, it is just a bit sleepy.
I am convinced now that the rough idle is being caused by a vacuum leak which I can't locate. Will have to take it to an expert for that.
Hopefully the above will help others as there seems such little info anywhere about this.
Adam
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