I had the same problem, had been waiting for 3 months and they failed to deliver on three of their delivery dates. If the delivery is 1 month later than they stated you have the right to cancel your order. I cancelled mine and found the same car at another dealer ready to pick up. In fact it had recaros and cd changer for the same price.
I would recommend ringing around to see if you can get the car somewhere else for better price or better availability. Then complain to your current dealer and try to get them to throw in, supergard, full tank of petrol or better part exchange on your car (if your part exchanging).
If you decide to go else where, then make sure you get your full deposit back before placing a deposit on the other car.
hope that helps