You are under no obligation to tell them where he lives or to assist them in any way whatsoever, I would also state that in your letters. Do not be too polite in your wordings and please do not worry about the DCAs. As far as you are concerned they are irrelevant. I fully understand that you are concerned about your son but purley on the debt chasing side they can not do you any harm and if your son has no assetts there is very little they can/will do to him either.
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Option One: Is as Sarah, states, return these DCA letters back to the DCAs who sent them.
If you have already opened them, do not worry, just put them back in another envelope and mark the envelope clearly:
RECIPIENT OF ENCLOSED DOES NOT LIVE AT THIS ADDRESS - PLEASE RETURN TO SENDER - POSTAGE WILL BE PAID BY ADDRESSEE ABOVE
Option Two: Write back to the DCA making it very clear to them that your son does not live with you and that you are in no way financially associated with him. Make it very clear that although he may be your son, you have no idea of his current address and that if they continue to send their letters to your address, you will report them the relevant Statutory Authorities, citing The Administration Of Justice Act in your defence as you feel that you are being unfairly harrassed by their letters.
Make it very clear that you will not be harassed or intimidated by them and add that if they continue to harass you with their letters relating to your son that YOU will take THEM to Court. Be VERY firm and to the point.
And, even though these letters do not refer to you, I would also demand that they send you a copy of their own Internal Complaints Procedure. I'm not sure that as you are not the debtor that they are legally obliged to send you a copy, but demanding a copy nevertheless, will show them that YOU mean business and will hang them out to dry if you have to.