This is happening with increased regularity and is indicative of a systemic failure within the CNBC. I am already dealing privately with 4 other similar cases. Three where no claim was received and one identical to yours where the defence was submitted but a default CCJ issued anyway.
As a matter of interest, in all the cases I am dealing with privately, the CCJ was received as a single A4 page from HMCTS with the CCJ (N30 Judgment for Claimant [in default]) printed on one side and the N1SDT Claim Form and Response Pack Cover Letter on the reverse. Is that the case with you?
If so, I am happy to assist in getting it set aside, at no eventual cost to you. However, I would need unredacted copies of the CCJ and the auto-response email from the claimresponses.cncb@justice.gov.uk. Also, when you emailed it, did you CC yourself and receive a copy of it? You can PM me if you want to join this group complaint to the HMCTS and I can use as much evidence of the systemic failure to make the claim stronger.