it wont work on disco 1....you past 85,000 miles or so? its probably just the emissions check reminder....you have to have it reset by the dealer...BUT there is nothing you need to change, they might change 02 sensors, but yours are probably ok still....just a reason to 'make' owners bring to a dealer...if you want to end it for sure, then remove the passenger seat kick panel (the one under the glove box), on the right side are a bunch of relay boxes, one is red and says Land Rover Emissions on a sticker on it, just unplug it, your service engine light will go off and you wont have to worry about it any longer!
edit*: as tommy33 said, you can reset by 'shorting' the two wires through the sticker, i opted that in case i ever sold it, id rather have the 'stock' un modified rely box in tact, without damaging the sticker and or whatnot...figured itd be easier to just remove the whole box, im never going to bring it in for emissions other than smog when needed at registration...