What are you paying currently?
Do you already have, or are there planned FTTP installs in your area within the new contract period?
£21/month appears reasonable considering it's £26.50/month for a new customer.
I feel like I'm about half a year ahead of you... I signed up for a £21 contact in march that was meant to end in September 24, party way through the year they then upped it to £64, I called them and they dropped it to £39. As if last month there charging me £59 and I can't find anywhere that tells me when the contract now ends. Calling customer services just gets me fed gibberish. I strongly recommend looking at the fine print, especially their options for raising the prices later way through a contract.