Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
My answer will be posted here and you will get an email to tell you so.
I'm assuming that the tax return is for the year ended 5 April 2015 which was due for submission by 31 January 2016. A further penalty of £300 will be added if it is not completed and submitted to HMRC by 31 January 2017. It strikes me as odd that you had no idea that you had been put into the self-assessment system and that you didn't receive the £100 penalty notice in March. Normally, a taxpayer registers themselves for SA if and when it is necessary as opposed to HMRC registering them. Have you moved home since April 2015? You need to ask when the return was issued and when and to what address. You also need to ask why you have been issued with one since you don't appear to think you need to complete one. Do you have any sources of untaxed income, rental income for example, a life policy gain?
I realise that you have called HMRC already and not been answered and that you have written a letter but the quickest way to get this resolved is to call again. You could wait months for a reply to your letter. Call HMRC on the number here and tell whoever answers the phone that you wish to make a complaint and that you will only talk to a complaints manager. Impress on them the urgency of the situation as you may have to wait for a call back if no complaints manager is available. When you do get to speak to one, explain your situation, ie that you were not aware that you been put into self-assessment and that you don't recall receiving a tax return and that if you had, you would have completed it or at least enquired as to why you needed to do so. The likelihood is that you will need to complete the tax return unless HMRC admit to issuing it by mistake but you won't know if you do until the matter has been looked into and HMRC have given you a reason for a tax return being required. The penalty won't increase until 31 January 2017. If you do need to complete the return, you can appeal against the penalties using the form here.
I hope this helps but let me know if you have any further questions. I'd be interested in knowing the result of your call to HMRC and your discussion with the complaints manager.
You could try the number here. I know its frustrating and I suggest you keep a record of the time you wait and the cost of your calls. The best time to call is 8am Monday to Saturday.
If you had a company car, there would be a taxable benefit unless you paid an equivalent sum to your employer to cancel the benefit. That may have triggered the issue of a tax return for 2014/15.