hmrc letter

hi i have recieved today in the post a letter from hmrc asking for my details of my incapacity benefit since 2008/2009 i have been on this benefit (invalidity benefit) since feb 1993 dose this mean that i am going to get a big fat tax bill, i was to understand that if you were on invalidity benefit before april 1995 this benefit was not taxable income is this true or false because i am so bothered about this matter can any one help please i am so confused.