They allow 5 days for 'service' and then, I can't remember whether it's 14 or 28 days for it to be returned. The N180DQ is an administrative document and even if you fail to respond, you will usually receive a court order reminder.
As you've stated, simply download your own DQ, complete it, sign it by typing your full name for the signature and send it in a single email but addressed to both, dq.cnbc@justice.gov.uk AND enquiries@gladstonessolicitors.co.uk and also CC in yourself.