Brockers_420:
Not an 18 month contract, what you agreed to was an 18 month minimum term.
Not sure about the notice method. The Terms & Conditions will definately be right, what you were told over the phone about the method and timing may or may not be right.
I never take what I am told at face value.
It's unfortunate that your use only lasted 18 months exactly.
Later edit:
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Sneaky, they used a funny font, but what it says at Paragraph 7 of the General Conditions is to call Customer Services if you want to cancel.
(unless you are a business customer)
Possibly you have been told a porky (though it doesn't say that CS can't then tell you to write). Will leave that fight up to you.