Unfortunately, the dealer will argue that you should have fully inspected the car's bodywork ahead of buying it (even if that was difficult). The Consumer Rights Act gives you a right to reject the car within the first 30 days if you can prove that it had a significant fault that was present when you bought it. The word 'significant' is key here - a few scratches and an unclipped bumper won't cut it. It costs a dealer a lot of money to take a car back (not only in lost profit but also the cost of preparing it for the forecourt again and it'll be worth less with an extra owner on the V5). This shouldn't necessarily be your issue, but it might explain their reluctance to give you a full refund.

As for the Auris, the 30 days thing sounds odd. If you liked the Yaris so much you wanted to buy another for your partner, would they refuse to sell it to you? I wonder if the salesperson is trying to push it back so it contributes to next month's targets or something. Try taking it up with someone higher up at the dealership.