Apex domains cannot have CNAME records.
Question and response
Question
dig bytemark.co.uk. NS
Answer
bytemark.co.uk. 3 NS c.ns.bytemark.co.uk.
bytemark.co.uk. 3 NS a.ns.bytemark.co.uk.
bytemark.co.uk. 3 NS b.ns.bytemark.co.uk.
Authority
The response has no authority records.
Additional
a.ns.bytemark.co.uk. 3 A 80.68.80.26
b.ns.bytemark.co.uk. 3 A 85.17.170.78
c.ns.bytemark.co.uk. 3 A 80.68.80.27
a.ns.bytemark.co.uk. 3 AAAA 2001:41c8:2:0:0:0:0:3
b.ns.bytemark.co.uk. 3 AAAA 2001:1af8:4c00:4:0:0:0:78
c.ns.bytemark.co.uk. 3 AAAA 2001:41c8:2:0:0:0:0:5
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0