The first question is whether it was openly sold to you as PPI - some customers found they had paid for PPI without even knowing that they had it. If you knew at the time what it was, then it depends on how it was sold to you. For example, how much information were you given? Were you told about the areas that were not covered by it (there are a lot of exclusions which many banks didn't mention). You should get out all the paperwork and see exactly what you were promised.

Did they let you know when the insurance would expire or that you would have to pay interest on it? And also, was it sold to you in a 'pushing' way, eg they tried to imply that you really needed it rather than just offering it as an option? For example, my bank actually called me a few times and tried to sell me PPI - I refused, but that would probably have been mis-selling.

You can find full details of how to check your status, and how to claim compensation if you have been mis-sold PPI, here.