Apex domains cannot have CNAME records.
Question and response
Question
dig 128-255.203.187.81.in-addr.arpa. NS
Answer
128-255.203.187.81.in-addr.arpa. 5 NS secondary-dns.co.uk.
128-255.203.187.81.in-addr.arpa. 5 NS ns0.grinton.net.
128-255.203.187.81.in-addr.arpa. 5 NS ns2.grinton.net.
128-255.203.187.81.in-addr.arpa. 5 NS ns1.grinton.net.
128-255.203.187.81.in-addr.arpa. 5 NS ns3.grinton.net.
Authority
The response has no authority records.
Additional
secondary-dns.co.uk. 5 A 194.4.173.1
secondary-dns.co.uk. 5 AAAA 2001:8b0:0:81:0:0:51bb:5120
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0