Neil's https security certificate has expired and his website needs a new one.
However, Neil's site also works unencrypted, so try connecting with http rather than https, as per this link http://www.stubmandrel.co.uk
Giant but! Although we trust Neil and his website, other websites supporting http and https at the same time are a security risk to the user, possibly as a side-effect, and doing both with a dud https certificate causes red lights to flash! It creates a loophole roughly equivalent to bellowing your bank account details through a loud-hailer, which is unwise if the local scallywag might be passing. No wonder bitdefender is upset!
Knowingly connecting to an unencrypted website is safe enough provided you don't do anything during the session to release logins, passwords, account details or personal information under any circumstances. While Neil's site is benign, some of the others are honeypots, deliberately designed to collect sensitive data with a 'man in the middle' attack, or similar manipulation.
Dave