Thanks for the reply guys.

It's not so much the cost (though it does seem high for a Focus), I was more concerned with them telling me that unless it was a Ford one it wouldn't work!

We have since picked the car up and had a chat with the mechanic/technician who has said that the cat is efficient enough to pass an MOT (so to my mind that means it must be OK?), but because the downstream sensor is "so sensitive" it thinks that there is a fault with the cat.

So is it possible to get a less sensitive o2 sensor or should I be looking to fit a MIL eliminator to that o2 sensor or is there something else that I should be looking at (I don't fancy paying £420 to replace what is basically an already efficient cat).

thanks again
Geoff