I checked the agreement I have with my car I have just now(no, not the Juke). If my dealer has not delivered the car within 21 days of agreed date then I can write a letter canceling the agreement if the seller cannot deliver the car within seven days of receiving my letter.
I would therefor have to wait till original date of delivry was due then write the letter and make sure it went by recorded delivery then assuming no hold ups with the mail I could walk into my dealer say, eight days later and ask for my deposit back because the dealer would then have broken the contract.
You have to be careful though because the seller may be a finance company and not the car dealer.
Hope this helps - you would have to check any agreement wording that you have on your finance documents whether you are buying outright or PCP or what ever.