Apex domains cannot have CNAME records.
Question and response
Question
dig mattress-direct.com. NS
Answer
mattress-direct.com. 5 NS ns-112.awsdns-14.com.
mattress-direct.com. 5 NS ns-1922.awsdns-48.co.uk.
mattress-direct.com. 5 NS ns-1735.awsdns-24.co.uk.
mattress-direct.com. 5 NS ns-916.awsdns-50.net.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0