Ignore them, the is nothing they can do unless you acknowledge the debt to them.
Trust me I bumped loads, best one was from British gas from 2006 I forget the name of the company but they hounded me to death.
I sold the house and the was £60 pound outstanding on the gas. I don't even know if I owed it or not. Note that debt will have cost them about 60p to purchase from BG.
I still get the odd letter now and again at my mums.
I am currently on the dodge job of about £15k only 2 more years to go and am debt free.
If you do not acknowledge a debt within 6 years its null and void, unless its a tax or is secured.
Debt is good, paying it back is stupid!