Well 2 years on we know its a [problem] called speculative invoicing. You can safely just ignore the letters as they have never taken anyone to court as they can't prove it was you so cannot be sure of winning. The more you engage them the more they will bully you. Just chuck the letter in the bin and forget them. For every 100 letters they send out they hope at least 1/3 will be scared into paying and thats how it works. Look whats happened to ACS now LOL.