Apex domains cannot have CNAME records.
Question and response
Question
dig thundermail.uk. NS
Answer
thundermail.uk. 4 NS ns0.thundercloud.uk.
thundermail.uk. 4 NS ns1.thundercloud.uk.
Authority
The response has no authority records.
Additional
ns0.thundercloud.uk. 4 A 149.255.60.1
ns1.thundercloud.uk. 4 A 185.53.57.60
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0