The offer terms for us existing customers says: "Offers for Fixed Price Plans advertised by TalkTalk are available to existing TalkTalk customers who have less than 90 days remaining within the minimum term of their old contract. Existing customers need to sign a new Fixed Price Plan contract."
In summary, if you're within 90 days of your end of contract date then chat to the Loyalty team and ask for the new customer terms that you see in the TalkTalk advertising.
Ofcom have made a ruling that requires Communications Providers tell us of the best broadband deal available in the 10 - 40 day run up to an end of contract and annually thereafter. That'll start officially from 15 February 2020. I'm expecting TalkTalk to write a Blog in Community on how they'll be following this ruling helping us to get better deals.