underpayment

i have worked 16 hours a week for many years, with 2 kids its all i could squeeze in. recently at the jobcentre while looking for more hours as youngest now at school. The personal adviser noticed that for the past 5yrs working part time, i should of received c.tax benefit and housing benefit. But i received no such thing, i had been paying full rent and c.tax for all those years, and running a house and 2 kids. I have informed my local tax office, and should be due £252 a month back for all the years i overpaid them! not very often you overpay people i know. What i would like to know is how far back can the backdate? and roughly how long will it take to receive my money back, as i can assure you that they were quick enough to take it off me every month. would be nice for xmas and to know now for the future i wont be struggling as much to make ends meet. do u think i would need a lawyer? i do have all documents and was paying DD every month so i do have proof i overpaid them. I just dont know much about this and local office are only telling me i will hear a reply through the post
many thank xx