I have had the email with the 'Get a sneak preview' message - that takes me to a page that tells me it's an insecure site (certificate error) so it won't load. It's address is -newmail.talktalk.co.uk/overview-tutorials- not the open-exchange one you link to.
However, that email came AFTER one telling me 'Upgrade Complete - log in for a tour'
I don't like the 'upgrade' but since I don't use TT webmail I'm not that concerned, I use Outlook to get the emails from TT.
The 'Portal' i.e home page is full of certificate errors.
I tried it on my tablet and that says the newmail.talktalk address is unsafe so I can't get in that way.
So, another blunder... but we're used to them.