You may be able to dodge the cancellation fee if you kick up a stink about the change in contract, but likely the swines have a term in the small print that they can charge you extra should you misinform them. Given that it's an honest mistake, you could try softly softly at first, then start a PR storm on twitter, if they are a reputable firm, they will want to save face online and offer to put you in touch with customer service who might have more powers.
From the DD guarantee, if they didn't notify you in advance you have grounds to demand a refund from the bank. That said, it doesn't waive you from the contract, and you might incur extra fees from the company for not paying by DD. If you have a contract, they could pursue you for even more than you have already paid.