Hi there, I have the same problem with the trip reset! I've got a 53 plate 320d and every other OBC function operates 100% ok, I can scroll through the functions using the left hand stalk. It too was working when I picked up the car as it was only showing 15 or so miles.i`ve had the car for nearly 2 weeks, and it not worked, it must have been reset just before i got it because it read only 12mls.
there are a few things wrong with the car, but i knew that when i got. the only elecy probs are trip reset and the outside temp sensor is missing, the rest is just body work.