Ok, on each wheel hub, the rears are about 10 o'clock looking at the wheel and the fronts are in the top swivel joint, there will be a black wire coming out, not the pipe but a bendy wire. Find that and follow it to its end at the hub, there will be the sensor. Firmly push the round sensor back into the hubs, it may only move 1mm or two but thats all it takes, do all four.
I found one that had moved out, the others were ok, they are held by a sort of springy clamp type thingy so can slide about a bit.
To read and clear the codes, google "Discovery ABS codes" i think, there are many instructions out there. Basically you need a pen and paper and a paper clip.
Find the diag socket, under the drivers side drop-cover, its got six pins i think. Then turn ignition on and wait a few moments, stuff paper clip in to short out the correct two pins, careful as there is 12v feed and earth in there too!
Then watch the ABS lamp, it will flash a sequence as listed in the instructions, this will tell you the possible fault. It can store many faults so once you have the first one, pull the clip out and wait for it to flash that code again then it will stop, this means its cleared that code. you now need to reinsert the clip and repeat until you get the all-clear code. The instructions will say "remove the ABS lampo relay", dont bother, it makes no difference as i have tried it both ways.
I believe if you search "meritor wabco ABS" or similar you should find the manufacturers data on it. It is also likely listed on this site too as its very popular.
Dave