Seems that too much has been done for any to be usefully undone. You have received £3K, better than the original offer (if I understand correctly). The forging of your signature was a bit underhand, but you weren't available at the time. I am surprised that the value of your car was not agreed when you signed up for the new one.

When you part-ex a car for a new one there is only a certain amount of haggling you can do - usually by getting a few extras thrown in, like carpet mats. If you feel your car's low mileage is a good selling point you could try selling it privately, when you would be able to apply more pressure to the selling dealer's price; or you could try another dealer who may be more generous.

My experience of dealers is that they won't call you unless they have to - you have to call them. And unless your car looks extremely tatty, cleaning it up is unlikely to affect its exchange value.

As long as the £3,000 is safely in your bank account, you've done well, even if the methods of the dealer were unethical. A seven-year-old, semi-automatic Peugeot is a big risk for a buyer, and the short journeys suggested by the very low mileage make it more so, not less.

Most of us on here wouldn't have paid £3,000 for it, so rejoice and move on.