I've just had an opportunity to try a different browser. Chrome works fine but Firefox comes up with the security error even though the certificate on talktalk.co.uk is actually in date.
If you are using Firefox:
Your browser checks the validity of certificates. As a temporary workaround, you can set the browser not to confirm the current validity of certificates.
Firefox > Tools > Options > Privacy & Security
Scroll down and untick Query OCSP responder servers to confirm the current validity of certificates
Hi Gondola, thanks for posting the results of your testing. We do have an incident open for this and it is being investigated by the business.