Well, it's not really a grey area in terms of responsibility. As stated in earlier posts, it's ultimately the responsibility of you as the taxpayer to ensure that the Inland Revenue receive the correct details, either by providing them yourself or being pretty confident that somebody else has done so on your behalf. Unfortunately "somebody else's" don't always do what they are suppposed to do or get the details wrong. If you are on PAYE you only have to look at the coding notice you receive just before the beginning of each tax year. If you have a company car and there isn't an entry against "Car Benefit" on the right hand side then it's wrong! If there is, then it's only one further step to check the amount is what it ought to be. Granted the Inland Revenue site can be a bit impenetrable but there's lots of other free car taxcalculators out there if you Google something like "car benefit tax".
Unfortunately, even if it is somebody else's fault that leads you to underpaying tax, it's still you that will have to pay it. Even more unfortunately, if you overpay tax then it's unlikely that it will be noticed, the Inland Revenue ratchet works in one direction only!