Hi Anthony - I'm a motor trader. Sounds to me like your Mondeo may have been 'clocked, but you haven't given enough information to be sure. When you buy/sell a car, you have the option of entering the mileage onto the registration document (V5) when you send it off to have the keepers name changed. DVLA then record the figure on the National Mileage Register. For example, let's say that the PREVIOUS owner of your Mondeo registered it at 60,000 miles. Your buyer does an HPI stating that the speedo now reads 40,000 miles - that's a discrepancy as it has gone 'backwards'. It could be that the guy who's interested in your car put the wrong mileage down (8,000 instead of 80,000) or he got it right but the owner before you registered it wrong (60,000 instead of 16,000). The best thing you can do for a start is get an HPI done yourself (just Google HPI Check Car) Make sure you enter the mileage as it shows on your Mondeo NOW. It shouldn't cost you more than �20. The HPI report will tell you about ANY discrepancies and should also list ALL HPIs that have been done on your car, by date and type. Dealer and Insurance HPIs are labelled as such, so a consumer check should be easy to spot. By comparing the dates, you should be able to see whether the guy that's interested is a Motor Trader (or might have one doing it for him. Sadly, if there is a discrepancy on the mileage, there's almost nothing you can do about it as you can't prove what the mileage was before you owned the car.
Hope that helps