Assuming its the £40k surcharge the info is there, you pay the extra for 5 years. No pro rata.

Vehicles with a list price of more than £40,000

You have to pay an extra £335 a year if you have a car or motorhome with a ‘list price’ (the published price before any discounts) of more than £40,000. You do not have to pay this if you have a zero emission vehicle.

You only have to pay this rate for 5 years (from the second time the vehicle is taxed).

And that's the list price including all optional extras added at the point of first sale. Not sure about those added afterwards though.

As per the DVLA tax checker, the surcharge expires on 30/06/2022, so as expected it will need to be paid up to then, however when it comes to 12 months for now, is it prorata for the rest of the time or is the full 12 months due even though there is only 1 month of it being due left.

So far as I can see it's payable each time the vehicle is taxed up from it's first anniversary (ie second year's tax) up until its fifth. No pro rata, it's just an additional amount.

I wonder what happens if you SORN it when the current tax expires and re tax it on 01/07/2022. Or if you cancel the tax, say by transferring the vehicle to your partner after 01/07/22, cop a refund, and then re tax at the standard rate.