If you have in writing that the estimated delivery date was the end of September then you should be ok getting your deposit back. As this is usually an estimated delivery date, the dealer would have 28 days from the estimate in which to supply the car before you can start the 'cancellation ball' rolling in respect of breach of contract.

The 4 months scrappage scheme is something that the dealership would need to worry about (they will have a way round this) if you still want to proceed with this order.

HTH