Agree with PP's, once past the cooling off period, and if the contact want subject to finance, then you're locked in.
However, it makes more sense financially for the car yard to let you out of the contract and to keep your deposit as it's really money for nothing for them. They'll get to keep your deposit, and then still sell the car to someone else for full price. So really its better for them if they let you out of it.
Hope that they're reasonable for you.