Maybe because its being delivered in a new finnancial year, the new car tax regulations are kicking in and they didn't think about it when selling to you. Sounds like they are trying to sting you with it now though. To me if you have agreed 'on the road prices' and signed a contract then they should take the hit!
If they try and argue the point tell them you are pulling out the sale and taking your business elsewhere - that may help them change their minds.