The certificates I see are in date. Click on the padlock icon and you'll be able to see more details about the certificates in use.
Do you see a grey padlock icon in the browser address bar, a yellow notification icon or a red warning icon?
Screenshot and post what you see please when (on Firefox) you select the padlock icon and then the > and More information to get certificate details.
Thanks for taking time out to let us know it's all good with you today.
There was planned website maintenance between midnight Sunday and 6am Monday so the main websites were flagged on Service Status as not available during those times.
TalkTalk Mail, though, was not expected to be impacted. Possibly the sign in authentication system needed a reboot after the maintenance session and that would be the common factor for your transient reports.
But good to know it was just a temporary issue.