Private vehicle sale

We bought a vehicle from the owner/privately,my problem is that the kilometres on the odometer is not the same as the kilometres from the service history of the vehicle. The kilometres are less on the dash than the info on the service history,the previous owner claims to have not tampered with the clock as they it is against the law. We have asked them to supply us with the previous owners details in order for us to get to trace the culprit. She has not sent or given us any information besides the fact that they bought the vehicle at a auction house. Can someone please help or point me in the right direction as we are not sure what to do next.