Edna cloud

Hi I bought a 4 year old c4 from a reputable dealer, 19k on clock, old mot matched new one. A month later halfords fitted a stereo and my air con and fan stopped working. They did a fix through autocentre fixed heater panel and replaced all fuses and control unit,removed the new stereo and put the factory stereo back in. It seemed fine till i noticed the mileage logged for car at end of job was 133k. Halfords are making me take the car to a Citroen dealer for a full diagnostic but the dealer warns me it will likely come back clear as other then dodgy odometer reading everything is working since it was repaired. Does anyone know how to get a true mileage for the car- so I can ask Halfords to do this? I know zero about cars and I still owe money on a car that now has dodgy mileage, and is worth squat...