Richard,
As far as I know you can cancel right up until you put your pin number in. I think its down to the garage if they will refund the money you have already placed on the car. I dont think the MPG of the diesel in the tiguan has a reputation for not being efficient on small trips. If the garage wont refund you then I think if you only doing 5000 per annum, if you lose the £500 then that would probably equate to more than the saving on a petrol car?