You have a contract, and it is the dealers duty to honour it, unless there is a specific legal clause regarding additional fees before final settlement.
If the dealer had undertaken the sale in a professional manner, then he should have allowed for all eventuallities like Vat.
The dealer as provided his governing bodies details, so contact them and see what their response will uncover.
The distance selling regulations can be slightly complex, as certain things can be excluded. For better advice, contact click here or download click here