I was wondering if what I have been asked to pay additionally by Virgin is legal.

I took a Virgin package (Landline & Broadband) for £15.50 a year back and the minimum contract period of 12 months got over on this January 2009.

They have started to charge an additional £5.50 pounds from the following month after the contract got over and (I failed to noticed this earlier as they get the payment automativally through Direct Debit) and I have recently called them and checked about this additonal charge and I was told by the customer service person that I was getting the pacakge on a reduced price of £15.50 as an offer during the contract period and now they are charging the standard charges for the package after the contract period.

I was not informed about this when i took the contract and also never got any mail from them informing this before they start to charge and just got the bill with updated charges (which I failed to notice earlier).

I never changed by package from the 1st day of service got activated and still continuing with the same package till date

Can somebody who is using Virgin services please advice me if this is a normally happens after the end of a contract or I can challenge them ?

Don't know about your particular contract but its quite clear at point of sale on the Virgin Media website.

The bill should have given the required notice for the Direct Debit increase.

Phone and broadband is a nightmare. You may still have the right to go back to a BT line without reconnection charge. If so there are some good deals if you are in some exchange areas. Check out PlusNet for example. If you threaten to leave Virgin you may well get a retention offer or more likely an offer to increase speed but not to save any money.