Billed for a cancelled contract

In the second week of January, I contacted Neotel telephonically to cancel my contract. I was advised that my contract would be moved to prepaid. No mention was made of any written cancellation. <br> On the 13th of Jan I received a mail stating that I needed to request the cancellation in writing. I duly responded (same day) with a short e-mail saying I wanted to move my contract to prepaid.<br> However, in February my account was again debited for the unwanted contract. I e-mailed Neotel on the 4th of Feb and they advised I'd never sent the written request. I responded, attaching the e-mail (which had been sent on the 13th of Jan). It is now almost a week later with no response and I'm pretty sure the debit order will again go off my account if I don't do the consultant's job and chase on their behalf!!!!!