Thank you both for your help

For the last year I have had 3 debt collection agencies contact me in relation to a gas supply to a flat I moved out of in 2005. The gas bill of £111.44 is in relation to a period of 2006 to 2008. The previous 3 DCA's stopped contacting me after I provided a court order evicting me from the property, a letter from the landlord confirming my eviction agreeing full and final payment, and a letter that I wrote stating the same. It is really starting to wind me up now, and I have had a new debt collection agency contact me this week with another demand. How am I able to get this harassment to stop? Do I have grounds to sue for harassment? Any advice is appreciated. The latest DCA is Wescot Credit Services. Thanks, Steve