Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question. You must now make your peace with HMRC and throw yourself on their mercy. Unfortunately that commodity tends to be in rather short supply in that Department.
State Benefits like Employment Allowance, now Job Seeker's Allowance, are mosty taxable, but paid gross so you might be in a spot of bother if they exceed your personal allowance in a tax year. Are you currently employed and paid through PATE? If so you will have a tax reference which you will find on your notice of coding. However, here is HMRC's guidance on to whom to write initially:
As you get in contact you can say what your income was in each of the tax years up to 14/15 and ask if they wish you to make a return for each of the years. I suggest that you do this in writing so you have a record of what you have said. It is possible that they may tell you not to bother, even HMRC do not wish to waste time and effort in processing returns which yield no revenue.
Unfortunately HMRC may charge interest on back tax, it is only fair for they give interest on over paid tax from prior years. They can also charge penalties, although their latest press releases indicate that this is no longer policy normally.
I do hope that I have been able to show you a way forward with your tax affairs.