Your car had done less miles that any "new" car I have ever bought !!
I would write them a recorded delivery letter straight away telling them that although you signed the agreement you thought that you would be paying for the car with the lower rate of finance as it wasnt explained to you properly (asuming of course the business manager there didnt explain it to you in detail, which I am quite certain he should have done - thats his job).
DO NOT have conversations with them on the telephone about this, you have no proof of what has been said.
Speak to Consumer Direct for advice asap;
Consumer Direct
Good Luck