there are two sensors, one being post cat and one being pre cat but £400 is fucking daylight robbery!!!

You literally unscrew the old ones out, screw the new ones in and reset the eml light!

The sensors are expensive if you get the from ford but I got mine off ebay for I think £30 or something for a genuine bosch sensor. Ask boschman on here (Ima Racing) he might be able to get you some.

And £400 is way over the top for 2 lambda sensors!