Apex domains cannot have CNAME records.
Question and response
Question
dig not-set-yet.virginmedia.net. NS
Answer
not-set-yet.virginmedia.net. 3 NS ns1.virginmedia.net.
not-set-yet.virginmedia.net. 3 NS ns3.virginmedia.net.
not-set-yet.virginmedia.net. 3 NS ns2.virginmedia.net.
not-set-yet.virginmedia.net. 3 NS ns4.virginmedia.net.
Authority
The response has no authority records.
Additional
ns1.virginmedia.net. 3 A 62.253.162.237
ns2.virginmedia.net. 3 A 194.168.4.237
ns3.virginmedia.net. 3 A 62.253.162.37
ns4.virginmedia.net. 3 A 194.168.4.33
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0