1. Dear *****, the first thing to be aware of, is that in law, you have notified the other party to your contractual agreement, Virgin Media, that you were terminating the contract. Phoning them and informing them by phone was sufficient notification in law. If Virgin Media's Terms & Conditions require notification in writing this would constitute an Unfair Terms under the Unfair Contract Terms REgulations and Directive and so such a requirement would not be enforceable.
2. Essentially, by informing Virgin Media that you wanted to be disconnected on 25/04, you brought the service agreement to an end from that date. Their authority to bill your account for any further monies ended from that date. So, in essence, Virgin Media attempting to bill you subsequently is unlawful practice.
3. You should initially make a complaint to Virgin Media. However, failing satisfaction from them, you should look at making a consumer complaint. Here is a link which will help http://www.consumerhelp.ie/how-to-complain?gclid=CNLT8ejF7M4CFeMW0wodCmUBHw
4. Make a complaint to the Competition & consumer Protection Commission http://www.consumerhelp.ie/report-a-business
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