Apex domains cannot have CNAME records.
Question and response
Question
dig tnp.net.uk. NS
Answer
tnp.net.uk. 5 NS ns2.tnp.net.uk.
tnp.net.uk. 5 NS ns1.tnp.net.uk.
tnp.net.uk. 5 NS ns0.tnp.net.uk.
tnp.net.uk. 5 NS ns3.tnp.net.uk.
Authority
The response has no authority records.
Additional
ns0.tnp.net.uk. 5 A 5.61.120.40
ns1.tnp.net.uk. 5 A 5.61.120.41
ns2.tnp.net.uk. 5 A 192.248.172.48
ns3.tnp.net.uk. 5 A 45.76.84.99
ns0.tnp.net.uk. 5 AAAA 2a01:5ac0:0:2:0:0:0:a
ns1.tnp.net.uk. 5 AAAA 2a01:5ac0:0:2:0:0:0:b
ns2.tnp.net.uk. 5 AAAA 2001:19f0:7402:512:0:0:0:53
ns3.tnp.net.uk. 5 AAAA 2001:19f0:6c01:2ab2:0:0:0:53
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0