Land Rover Repair: SES Light - possible bad O2 Sensor?, o2 sensors, maf sensor

Question
Hello ,  I have a 2004 Discovery II with about 59K miles on it.  At 56500 miles the SES light came on and the code was P0134. I know this is sensor 1 bank 1 O2 sensor.  I read that the O2 sensors are usually replaced around 60000 miles and I had a road trip coming up so I ordered and replace all 4 of them. The problem O2 sensor was ashed over very grey and the other 3 were a sooty black.  I figured this was the cause of the code so I reset and all was good for about a day then the ses light came on again. The same P0134 code came back showing it was not responding. I also decided to replace the MAF sensor as well and did so with a genuine land rover part.  
I had to leave to go out of town with the disco for about a month.  By the time I drove the 6 hours back it was running rough and code reader still showed P0134 as well as cylinder 5 misfire(I forget the code number).  Today I replaced all of the plugs as I am withing a few hundred miles of the 60k service.  Number 5 was a very dirty, fouled plug.  Now it runs very well but I am still getting the P0134 sensor 1 bank 1 code.  I checked the connector today after doing the plugs and it is connected properly.  There is no signs of burnt, cracked, or grounded wires.  The connectors are clean and in good shape with the wires showing no sign of being pulled out of the connector.  I am at a loss.  Not sure if this is a fluke or what.  Sensors were purchased from Brit Atlantic.  This is the link for what I bought, http://www.roverparts.com/Parts/MHK100920.cfm .  Is there something I am missing? It threw a code for the sensor(Which appeared to be bad), I replaced the sensor, and still get the same code.  Could it be something further upstream from the O2 sensor, and what might that be?  Any help or suggestions would be great.  Thanks in advance,

Thomas

Answer
The problem with your car is probably escaping detection because you are relying on fault code data alone.  We (or any shop with a real LR test system) would read the live data stream, which would let us see the action of all the sensors.  That would probably tell the tale.

Static fault code data only takes you so far, as you are discovering.

My first suggestion is to find a shop with a T4 or Autologic.  Failing that, you could read the o2 and MAF sensors with a lab scope, if you knew what to look for.