Yes you actually had a few issues to resolve.
1. BT Web Protect with a warning page to the site - you have now disabled this through MyBT for your connection.
2. The domain itself is actually on a malware/malicious list
- Flagged by Google
https://transparencyreport.google.com/safe-browsing/search?url=defendnattack.com&hl=en_GB
- Flagged by Fortiguard
https://www.fortiguard.com/webfilter?q=defendnattack.com
You will need to resolve the reputation of your domain as it could cause issues for you further down the line if it ends on blocklists and safe/protection systems.
Agree with @jac_95 advice, also look at your SSL certificate, Firefox and Edge flag it as an issue.
Is it self certified ?you can get them for very little these days.
@-Richie- looks like there's no redirect from http to https which is why you see a certificate error as it's not using the secured connection. Can be configured at webserver or ALB level
@jac_95 Good spot, manually entering the HTTPS works, without it gets flagged as insecure.
thanks all for the help.. it seems that the BT issue has cleared and it is now a safe site issue with the HOST company (fasthosts) and also with trying to clean up where it has been flagged as unsafe (which could take months reporting it to all these different companies)...
once again thanks for all the help.