[ergonomist]

Most standard purchase agreements for a new car include a clause that allows the dealer to pass on any increase in cost that is imposed on them (whether by Manufacturer or due to changes in Govt duty etc). The clause usually gives the buyer the right to withdraw from the agreement in the event of such an increase.

If you want to be sure that no surprise increases will be imposed, you should get the relevant clause removed/amended before you sign the agreement (which the dealer may be happy to do in order to secure the order)