Not often we see a question from you, buildersmate.
I think I understand this although I'm not sure how it turned out teh debt related to 22 Acacia Avenue not Flat 6 assuming no payments were ever made for the utility service at Flat 6.
Anyway, the utility company may dispute that they were ever given Flat 6 as teh address - is there a written record or recording of phone call of the original change of address notification? And the utility company might say Dave should have queried the fact that he wasn't getting any bills or paying anything whilst at Flat 6.
If it can be shown to have been a utility company error I think it's their responsibility to call off the debt collectors. I would try again with the utility company (maybe enclosing a cheque) and say Dave will go to the regulator- or his MP if it isn't sorted.
There may be a legal letter you could write to the debt collectors to stop harassing Dave but they will just say they have bought the debt correctly so it's not their problem.



