All of your bills advise the end date of any discounts. As does your contract. If you have online billing then you can check your bills and contract by signing into My Virgin Media at the top of the page. An automated email is also sent advising of payments and contract discounts ending with a reminder of the new package price. This may be in your spam/junk folder.
You can contact VM and negotiate a new contract.
You could try the text messaging service. Just send a text with a description of regrade to 07533 051809. Your message will go into a queue and a representative will respond to you as soon as possible. It may take a few days.
You can also call 150 from a Virgin landline, or 0345 454 1111, options 1,1 and 4 (Changes to your package) butbest to call at 8am to avoid call queues
The Service you do for others is the rent you pay for your room here on Earth - Muhammad Ali