Apex domains cannot have CNAME records.
Question and response
Question
dig tertiary-mx.co.uk. NS
Answer
tertiary-mx.co.uk. 5 NS primary-dns.co.uk.
tertiary-mx.co.uk. 5 NS secondary-dns.co.uk.
Authority
The response has no authority records.
Additional
primary-dns.co.uk. 5 A 194.4.174.1
secondary-dns.co.uk. 5 A 194.4.173.1
primary-dns.co.uk. 5 AAAA 2001:8b0:0:30:0:0:51bb:1e29
secondary-dns.co.uk. 5 AAAA 2001:8b0:0:81:0:0:51bb:5120
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0