We were coming up for our 6th (november) contract i think, but as by the end of the year we will be off on a one way ticket, a certain contract needs to be cancelled.
Its easier to phone 111 - you will get a sms confiming the cancelation date. You need to give one calendar months notice and the intial period must be complete i.e. all 24 months meaning the contract is in effect a 25+ month contract.