Apex domains cannot have CNAME records.
Question and response
Question
dig zen.net.uk. NS
Answer
zen.net.uk. 5 NS ns1.zen.co.uk.
zen.net.uk. 5 NS ns0.zen.co.uk.
Authority
The response has no authority records.
Additional
ns0.zen.co.uk. 5 A 212.23.8.1
ns1.zen.co.uk. 5 A 212.23.3.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0