Virgin Media — Over Charging

I have been charged for my telephone line in addition to my Broadband package even though the telephone line was discounted when I took out the service -as it is still discounted now. Virgin have adjusted my billing going forward but have refused to pay back the money they overcharged for previous months. They are basically saying that if I did not notice that they were overcharging me then its my fault and they can keep it!

What on earth can I do about this? If they have simply made a processing error and charged me more than I agreed to when I took out the contract how do I defend myself and get this money returned? For starters, how do I prove what the charging was agreed to be?