Updating your V5C now will not stop further letters about this going to the old address. The only way you can change the address for service is to complete the Acknowledgement of Service (AoS) with the correct address for service. Without knowing what the issue date of the claim is (please show us as advised in the previous post) but assuming that it is no earlier than 8th January, the you should submit your AoS according to the instructions in this linked PDF document:
https://www.dropbox.com/s/xvqu3bask5m0zir/money-claim-online-How-to-Acknowledge.pdf?dl=0However you MUST also send a Data Rectification Notice (DRN) to the DPO's of both ECP and DCB Legal. The DRN must instruct them to
update their records with your
current address for service and to
erase your old address. The highlighted words are there for a reason and you must use them.
Having submitted the DRNs and the AoS, you will then have to submit your defence. Until we have seen the Particulars of Claim (PoC) (even though we already know what they state) and you've told us the date of issue of the Claim, there is not much more we can do. We will provide a suitable defence once we know that the AoS has been submitted and the deadlines are all OK.
No one pays a penny to ECP if they issue a claim through DCB Legal as they will eventually discontinue as long as the claim is defended.