A while ago while changing struts, I noticed this sensor unplugged, which I assume has been since I got the car. It's a 2 wire sensor but the plug looked to have 4 pins if I remember correctly. At the time we wired it in so it wouldnt throw a CEL but I never noticed a difference to when it was out. Anyway just wondering what it is and whether it would make a difference replacing it. Cheers

I think it might be the post-cat oxygen sensor (the main one is pre-cat).
I wouldn't be surprised if it made no difference having it unplugged.

hmm, i think you have the loom for a 4 wire O2 sensor, but it is perhaps connected to a cat temp sensor.
 
you shouldn't get a CEL without an O2 sensor plugged in.

Oh yeah it could be a cat temp sensor, or what I said. I'm not sure.

I originally thought it was an o2 sensor also. I tried looking at some diagrams and it looks like it might be a temp sensor. Is it worth worrying about? 
 
 

nope. it's wired up wrong either way. should be able to disconnect it. cat temp sensor is pretty useless.