It's a Trading Standards case. The potential criminal case is separate from the civil issue. On the civil issue it depends on what outcome you want to achieve - specifically whether you want to keep the car or not and whether you can locate the trader and they are solvent.

Well he wouldn't would he?

My advice would be to change the vehicle as if it has been clocked you don't know what you have got.

All the "service history" is worthless and you will have to sell on with a statement regarding the speedometer or you would be committing an offence. HJ would be able to comment on what effect this would have on the value of the car but I wouldn't buy it.

Assuming the dealer was an "innocent party" (or wants to prove his respectability) you are in a strong position to negotiate a good deal on the replacement.