I just checked... A recent 18 month contract renewal with Plusnet gave me line rental, and unlimited broadband for £19.99 a month, but as I mentioned before, I get a reduction for signing up a couple of people. I am actually paying £16.24 a month.
They will sneak that back up to £31+ / month at the end of the 18 months. I just get in touch with them a month before the end of the contract and say that I am thinking of going to TalkTalk (whoever) and they immediately offer me an 18 month extension at not much more than the price of the previous contract.