Hi Charles,
When placed my Order Hartwell Kiddlington, the Copy of Order / Receipt for £1K Deposit specifically says price correct, but any Government Charges will apply at the rate applicable when car delivered.
So unless your dealer is very generous I think you will have to stump up the extra, but, The yearly amount reduces after first year.
Earlier threads have calculated that you will need to keep car 5 Years to reach parity with what is being paid today.