compliance letter

Hi, I have just received a compliance officer letter asking me to attend an interveiw, as i am not on any income related benefits i phoned up to ask what it was for and was told they could do it over the phone at the time stated on the letter.what i would like to know is do i have to disclose all my savings, pension etc to them, as stated i am not on any means tested benefit and why after such a long time on invalidity , incapacity, and now in E.S.A. support group do i get my first one of these. Also as it came from D.W.P. i found it very strange that my national insurance number was not on the letter. Could it have been sent after somebody being malicous making phone calls to D.W.P. or is this just the norm. Many Thanks. Macca.