Some traders fit an electronic chip that converts the input signal to the speedo/odometer from kmh to mph. They also black out the K on the speedo so it just shows mh. You should have had some paper work confirming what the odometer reading was before and after the chip was fitted. Going by your figures, you have done 12,000 miles, is that correct? It could be a problem to some people as they might think the odometer has been fiddled with. For your own peace of mind, put your chassis/vin number in on the CarVX site, pay the fee approx £20, they will then email you the auction report showing the odometer reading at the time of the auction plus any damage, recalls etc. Some chips can fail and cause various faults with warning lights etc.