Apex domains cannot have CNAME records.
Question and response
Question
dig ntp0.cam.ac.uk. NS
Answer
ntp0.cam.ac.uk. 5 NS auth1.dns.cam.ac.uk.
ntp0.cam.ac.uk. 5 NS auth0.dns.cam.ac.uk.
Authority
The response has no authority records.
Additional
auth0.dns.cam.ac.uk. 5 A 131.111.8.37
auth1.dns.cam.ac.uk. 5 A 131.111.12.37
auth0.dns.cam.ac.uk. 5 AAAA 2001:630:212:8:0:0:d:a0
auth1.dns.cam.ac.uk. 5 AAAA 2001:630:212:12:0:0:d:a1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0