I asked again and was told the same registration of the vehicle is only completed when the vehicle has been manufactured with a serial number and available to assign a number plate and VAT reg.
I was reassured that the offer has been running for 2 years, as soon as bmw came back on motability this year the offer came back on so more than likely it will be there next year.
its whether your willing to take the risk or not to save the £500 cashback.