Not an expert but I'm assuming this is a DCA chasing you in respect of water rates.
Pretty sure that the CCA doesn't apply in this case but you should have signed some sort of agreement authorising them to supply you water.
I'd Subject Access Request the water company ASAP and send a suitably amended CCA letter to the DCA requesting the signed agreement and informing them that you are in contact with their client disputing the amount being claimed on the account, (don't ever use the word debt) it may contain penalty charges for instance.
Data Protection Act - Subject Access Request Generator *** New ***
Creditors and DCAs - Letter Templates & Budget Planner
Amended Letter N
I'd also post this on the utilities thread to be with peeps who understand this sort of thing.
Bottom line this is the DCA firing a warning shot and if they get a panicked response from you they'll be in for the kill.
Subject Access Request'g the utilities company & requesting the agreement or proof that the account is in arrears from the DCA will tell 'em that you're not a no nothing 'easy mark'
rgds
MAC