Apex domains cannot have CNAME records.
Question and response
Question
dig interdsl.co.uk. NS
Answer
interdsl.co.uk. 5 NS ns4.interdns.co.uk.
interdsl.co.uk. 5 NS ns3.interdns.co.uk.
interdsl.co.uk. 5 NS ns1.interdns.co.uk.
interdsl.co.uk. 5 NS ns2.interdns.co.uk.
Authority
The response has no authority records.
Additional
ns1.interdns.co.uk. 5 A 83.170.124.44
ns2.interdns.co.uk. 5 A 173.244.219.15
ns3.interdns.co.uk. 5 A 35.177.60.186
ns4.interdns.co.uk. 5 A 45.93.112.252
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0