it's the responsibility of the owner to sorn a vehicle if it is not to be taxed for a while ,so from august to november it would have been the sellers problem. it stays the sellers problem until it is registered in your name so from the date of purchase you would be responsible. you may end up with a fine for the 4 months it hasn't been sorn but if they say it had to be taxed from 16.08 2006 i think you could argue that you bought it in november and should only pay the road tax from then. i would do what ' thames' says and write to them first, then it looks better on you for enquiring. good luck