I've just picked up my car from the dealer in Bristol having ordered it on 01 August. I was concerned at the time of order that the £2500 I got off for the scrappage scheme may be at risk if the scheme was stopped before the car was delivered. The dealer confirmed that you enter into an agreement at the point of order which both sides have to honour.
I also see that most cars have gone up £500 on 01 Oct and there was no attempt to pass this on, hence I'm not sure why VAT would be different. Devil is clearly in the detail however and I was never given this as my order form did not have this on. I would ring several SEAT dealers and confirm their position and if you still get no joy, I would ask for the contract and check with Citizens advice.