i've used this before and it is genuine
the only thing is it will take a few days to update,
i used it when i purchased my road tax online
another tip is, if you go to the post office they will not renew your road tax if for example your car is registered to your wife and the insured is only you to drive, but if you buy road tax online, they will issue the road tax as long as the vehicle is insured.
also this will not show vehicles that are covered under fleet policies