If you have not acknowledged this debt in the last six years (five in Scotland) then it becomes "Statute Barred"-- this means it cannot be enforced in court.
Reading the following link will explain it -----
http://www.nationaldebtline.co.uk/england_wale s/factsheet.php?page=25_liability_for_debts_an d_the_limitation_act
Read it carefully, and DO NOT give your phone number to the DCA , they will forever phone you.
Although they cannot take you to court to enforce payment, they can send letters etc , as long as they dont harras you.
Send the Statute Barred letter and then just ignore any further correspondance.