Sims

Hi Robbie,
                  I have a LE500 but I have looked at the pre SAIC MGTF manual and it should be in the same area. As you have an 04 plate the there should be pre and post cat o2 sensor.
I have copied the page from manual.
Have you followed the wires upwards from underneath the car where they cut it off assuming they are still there.
There should be the remnants of the harness if they pulled the whole o2 sensor wiring out with the cat. 
Sorry can’t be of much more help
Good luck

Simon