Apex domains cannot have CNAME records.
Question and response
Question
dig 4.80.in-addr.arpa. NS
Answer
4.80.in-addr.arpa. 1 NS ns4.virginmedia.net.
4.80.in-addr.arpa. 1 NS ns1.virginmedia.net.
4.80.in-addr.arpa. 1 NS ns2.virginmedia.net.
4.80.in-addr.arpa. 1 NS ns3.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