There was a long article on this in today's newspaper, seems like an excellent deal if you meet the criteria.

You must have had the car on the road for a year, not sure if the one you are trying to start qualifies? It must have been registered and insured for at least one year prior to the trade in.

They refer you to the on line Kelly Blue Book to see if you car is more valuable than the alllowance.
Of course, I can't find today's article on line.