my wifes ford focus 1.6 ghia on a 03 plate has the diagnostic light on,our mechanic mate pluged the computer into it and it came back saying its the sensor post cat,is this the hego sensor? ive been quoted about £130 for the sensor and the seats and carpet have to come out to fit it,i used to work at visteon in dunton and have lost touch with all this technology

you do not say what code was in the ecu, but before buying a new sensor do a few more tests.

With the cat hot, if the post-cat oxygen sensor signal is fluctuating in synch with the upstream one then the cat is gone and a code appears.
sample the exhaust gases to confirm cat failure?
did they measured the in and out cat temperatures?