And you are perfectly entitled to your opinion and tampering with the mileage is 100% wrong , however there are far more pressing things going on in the world right now to be overly concerned about an 18 year old vehicle ,that you don't even own, and has had a bit of a "haircut" in the last year or so - IMHO.
Submit your inspection report , with the mileage , and a copy of the
Autotrader Ad to the NMR +
Autotrader and leave them to get on with it , then you can sleep easy safe in the knowledge you have done your bit as there is not really that much more you can actually do.
Have you thought about contacting the dealer to let them know you have proof the vehicle has been clocked ?
Out of curiosity , what stopped you going ahead with the purchase last year ?
K