The odometer on my BMW X5 shows a different mileage from the key when the keys is inserted into the main agents key reader

Couple of things could be the issue. Does your cluster have a little dot next to the mileage. If so that is called the manipulation dot basically meaning some one has done something to the vehicle instrument cluster like change the mileage or install one from another car. If it's installed from another vehicle it could that other vehicles mileage. The miles on the e53 are stored in two places in the vehicle, in the instrument cluster and in the lamp module. If those two are not exactly the same then it sets the mentioned above dot. The other is the key it self is not storing the data properly. Those are just a couple things in that system as you can see it gets very involved fixing and issue with key and actual mileage. Hope this helps.

Robert has some great points. You might also want to check it the mileage is jumping back to Kilometers, which should be pretty close to what you are indicating here. It's worth looking into it.

Thanks to both Robert and Michael. There is no manipulation dot at all Robert, and I think if the key was in kilometre mode then the 52000 miles would be about 83000 on they key which is an interesting thought but have you ever had any direct knowledge of that having happened. The agents were unable to answer my question. There are companies specializing in what they call corrections of the odometer and it may be that the BMW agents do not wish to acknowledge that this can be done because they said that if you try and turn it back or manipulate it then all the readings go completely haywire.Thank you both again anyway. colinhere