Both my downstream Sensors failed, probably when I was messing around with my new De-cross pipe (I wasn't the most gracious when undoing them).
So I got recommended a part number, almost half the price of the genuine articles. (Can be found on the alterative parts list)
Now these are a pain in the arse to fit, and if I had to do this again I would 100% pay someone with access to a ramp and the correct o2 sensor tools, neither of which I have. But after a long 2 hours, maybe a few tears and a little less skin on my hands they were fitted.
The replacement cable was a little longer, but that's nothing a cable tie or 2 couldn't fix (plenty are included with the sensor) and the plug head was a different colour. Other than that they fit and work exactly as the original ones and a real bargain at £47