Your best of doing it through chat and here is the link to the page where the offers are.
https://sales.talktalk.co.uk/product/tv/plustv
It is about half way down the page. Also make sure you email yourself the transcript of the chat so you have all the details in writting. To email it to your self just click the big + on the chat window and that gives you a few options including email a transcript after the chat has ended.