The offer will be for 12 months, then reverting to the full price of the product, Infinity 1 is around £21 per month and Infinity 2 around £30. Whereas standard Unlimited broadband is £19.20.
There is a consumer options team with access to retention deals, just call 11 months in to the contract (you pay in advance and dont want to wait til 12months or you may get a bill at the higher rate).
I did it last week. Rang up the "cancellations number" as it was the easiest one to find on the website. Told them the deal I had arranged last year was coming to an end. She checked it all out and offered me discounts on Broadband and a new TV contract. It was good enough for me (I may have been able to hold out for better but probably not much better). Took about 15-20 mins in total.