Who is responsible ?

1. The previous owner. They are supposed to retain and destroy the old tax disc. A few years ago, you actually had to send the old disc back to DVLA for destruction.

2. The garage, for not checking that the tax disc matched up.

3. You. For not checking that it was all correct.

As to what is taxed ... both. The tax rate is based on the vehicle, the tax is on the vehicle bearing a particular number plate which is on it, and on DVLAs records.

What I suspect has happened is this : End of last year/early this year, the backlog with DVLA for private plate transfers was horrendously long. The car was part-ex'ed in, and when the transfer came through, the tax disc part was forgotten about.The old owner forgot about destroying the old tax disc, the person at the garage didn't know they had to destroy the old tax disc, and basically it all slipped through the gaps. And then, when you bought the car, you never checked that the tax disc details matched up with the details on the car.

So who is responsible, in reality ? You. You are responsible for ensuring that you are complying with the law. Ignorance is not a defence in a courtroom.