You should not have a CNAME record for www.sheltontrophies.co.uk. You should see a NETLIFY record the same as for the apex domain (read: [Support Guide] What are the NETLIFY and NETLIFYv6 type DNS records? How do I delete these records?)
If you followed the assign a domain to a production site documentation then likely both the apex and www subdomain were assigned.
If you don’t see the NETLIFY records for the www subdomain in Netlify DNS then un-assign the domain from your site and start the procedure again using the documentation above.