What Modem have you got?
Try our Speed test and post the results:-
http://www.cableforum.co.uk/board/speed50.php
Consider that at peak periods you may see lower speeds, also if you have been downloading a lot, you may have been traffic managed which can reduce your speed.
If you do not think the above applies to you and If you are seeing these pitiful speeds consistently, I'd get back on to Virgin Media.