I'm not going to have a go at the car, but I'd like to know why, if the low monthly payment was so important, you didn't just take a loan out for it from the high street banks?
£500 of your cash then £6.5K borrowed over 5 years is less than £135 per month at a typical 8.8% APR. The saving would have paid for the road tax over the time you intend to keep the car.
It was easier and less effort, that is the truthful reply. A saving of £10 a month isnt really worth messing about with