Clocking a car or supplying a clocked car is a Trade Descriptions Act offence against the dealer. You do not have to prove they knew about it or did it, it is said that they did and down to them to prove they did what they could to check mileage was correct.
I would say you have civil rights against the dealer for selling a car not as described, which would entitle you to give the car back and cancel finance. Would also be worth contacting finance co (as long as finance arranged via garage not your own bank loan) as they could well be jointly liable.
There is also the criminal side which Trading Standards would deal with, although is up to you whether you want them to look into it, or want to sort your own problem out first.