discovery 2 v8 o2 sensors

hi have to change the o2 sensors before the cat.
can you get these off from the top of the engine bay or underneath as it looks pretty tight.
any body done this job and how difficult is it to do.

thanks

Change them from underneath,its not difficult.Why are you changing them ?

done mine underneath no problem are a bit tight no used a bar on mine

hi reason i want to chnage them is i think one has gone or is on its way out.

1 side of the engine is running lean the other bank is rich.
when you put your foot down it tends to be missing.
have changed plus and checked leads.
some times its not there at all but seems to be short on power when cold, any body any clues as to whats going on.
did you use a socket or a special wrench to get them off , just did not look as if you can get the pull on them.
thanks