Help Regarding Charge for Unreturned Equipment

Hi 

Recently left virgin media, have received an email saying not returned the equipment and that I have been charged £40, they have sent me a payment ref number account number to use to pay this. However the payment ref number does not work? 

What can I do to actually get this sorted without having to spend hours on the phone? I did notice the payment ref number and account number they sent requesting I use to pay the £40 does not even match the account number I have from my old virgin media bills?

Any advice or help would be great thanks.

Really dont understand how it can be so difficult.