A car's ECU now records a car's total mileage, engine revs and road speeds in much the same way as an aircraft's black box clocks up total flying hours. This ECU recording is evidential and drivers have even been prosecuted for speeding on the basis of the evidence in the ECU. So if the car was sold as having a lower mileage than the mileage clocked up by the ECU it was falsely represented. MoTs are less reliable because a car can be run 'unhooked' (with the speedo disconnected) in between MoTs so the speedo and odo do not record the miles. This is done with Porsches because their value is very mileage sensitive. So you have a very strong case that the car was at least misrepresented and at worst clocked on the basis of the evidence in its ECU. And the ECU may contain other evidence such as the engine having been over revved, etc.